Rock on Ken, sounds like you've got some great opportunities to step up to doing paid professional work for a great community of people, soak up the fun and attention!
There's no shame in doing work for free or for starting to charge now - it's your camera, your talent, you can play the game any way you want. If you've got people knocking on your door so to speak wanting to pay you for work, that's a great sign that it's time to step up, if you have the time and interest.
I won't disagree with the depth of Keith's suggestions, but I'll offer a simple alternative: figure up your pricing based on how much you want to put in your pocket in trade for an hour of your time. There is no perfect price point, too many variables influence where the sweet spot lies, but you probably have a good idea of how much you'd like to take home from an hour of work. That might be $10, that might be $100 or $1,000 - balance what you want to earn versus what you realistically think folks will pay, of course. A happy medium isn't too hard to find. Just make sure the number puts a smile on your face.
It sounds like your potential clients want an all-digital service, which keeps expenses/overhead minimal and simple. Whatever hourly 'rate' you come up with, keep in mind that eventually you'll have some expenses: your shutter will blow, you'll want to upgrade your kit, you'll probably need some help from a CPA or tax preparer for figuring up all this business income, you may want to do some advertising or pay a web designer to hook you up with a nice site. Figure that's going to come out of whatever hourly rate you earn, and increase your rate according to your ambitions and future expense expectations.
You've been shooting for free, so money-wise, there's nowhere to go but up. Again, there's no shame in asking to be paid for your time and talent, any potential client who doesn't appreciate this fact isn't worth having as a client - let them take advantage of someone else's kindness.
Be humble in your pricing - you can always raise it later as your reputation and talents grow. That said, get yourself paid - again, make sure at the end of the shoot, the month, the year, after all expenses, you still have that canary-eating grin on your face.
Once you have an hourly rate in mind, just structure your shoots to try and hit this hourly rate, or better. Although I don't generally recommend packages to my fellow portrait photographers, I think your clientele would appreciate having some tiered options, especially ones that involve groups and entire clubs.
Such as, on the low end, you might offer a one-hour, on location, one- to four-car shoot for $XXX, to include shooting individual images of each car, and some group photos. The drivers get to split the cost, and each walks away with one individual hi-res digital file and an agreed-upon single group photo file. You can then upsell other individual images from the shoot, other group shots, even prestige prints like 20x30 wall hangings. Always try to give your clients the reason and opportunity to spend more money with you.
You'll have to think about the different ways individuals, groups, and clubs may want to invest in your photography. You could do individual prestige packages for the big spenders who love their cars, including hours of shooting time, multiple locations, images on CD and prints or groupings for display on the wall. You could do club-size bulk deals, even set up an annual photo event with them where every member gets a photo, and you set up an epic all-members group photo (think like a Team & Individual photographer here, volume work at lower prices but faster turnaround per subject), then get them to work the cost into their annual dues ($10 or $20 per member times a 50-member club is nothing to sneeze at for a few hours of shooting). You could have packages for smaller groups of friends, for cinematic Fast & Furious style shoots, and so on. You could have gift certificate promotions marketing to the parents or girlfriends or boyfriends of drivers, so on and so on.
The end goal is to always at the least hit that hourly rate that makes you smile. Don't forget to include the time you spend servicing the client before and after the shoot, and the time you invest in your web site, blog, processing and posting, etc. I've gotten very efficient at this over the years, but even still, for a one-hour high school senior photo shoot for example, I budget four hours of total time invested in the shoot.
Sounds like you have a few different opportunities to monetize your work - sales direct to drivers, leasing images to commercial entities, and advertising through your web site.
If you have the mental and business dexterity to juggle all three balls, you can maximize your income from each - but you can also just focus on one target market (sales direct to drivers, for example), and use other monetization opportunities to bolster and promote that specialty.
For example, a great relationship with the car shop owner you mentioned opens a lot of doors. Instead of milking him for "the standard commercial industry rate," you could barter and find other ways to build your exposure via his shop and contacts. Look for ways that you both benefit, let him keep his money in his pocket, and focus more on building a great relationship and getting your work in front of his clients and contacts. $50 for some files to print and hang in his shop, even $300 for six months of advertising on your site, that's peanuts - even just working part time selling direct to drivers, you should be able to make that much money in less than a day's worth of shooting. I'd say the better play here is to work with the shop owner on a mutually-beneficial trade that gives him great photos and exposure and gives you exposure and introductions to important people in the industry.
Think more along the lines of sponsorship, cross promotion, "official photographer of..." status, joint marketing campaigns and events, invitations to industry events, big promotional prints hung in the shop with your business cards, coop contests, etc. A great relationship with a popular, connected vendor like this can be worth far, far more than a few hundred dollars over time.
To make a short story long - Ready, Fire!, Aim. Set a price, do work, hustle, make money, measure your time versus your take versus your expenses, and adjust accordingly as you get a good feel for how all these parts work together to create your bottom line. Have some fun and make some money!