Personally, being at the same level experience and gear-wise that you are, here is what I would do (and have done):
Take the money she is willing to pay you and use it on renting gear/ any additional expenses. For example, say you will spend ~10$ in gas to get there, ~5$ in CDs/DVDs, and the other 380$ use on lenses (save 5$ for a drink or two.. don't want shaky nervous hands!
). Take this opportunity someone is giving you to full immerse yourself in the task. Sure this way you'll walk away with +0$, but the thing to focus on is the experience you gain, not the money. Use the money to bring her the best product you possibly can, and thats by at the very least renting the 70-200/2.8 and a good flash.
Rates Photographers are paid per hour are a reflection of their skill, experience, and investment in gear. You and I have hobby-level skill, very little experience, and a hobby-level investment in gear, so if you look at the situation as I do I would equate my hourly worth to.. well lets say its not 130$/hr (based on a 400$fee for the event). Instead of looking to walk away from this by getting paid in cash, look at it as a payment in experience and spend any money you would get for your time to optimize the final product.
I while back a buddy of mine looked through pictures I've taken of my sister's dance performances and asked me to go take pictures of his band performing. This is NOT the equivalent of a wedding, but it was important for him to have performance pictures to properly market and advance his band. I asked him how much there was in the band's budget for a photographer and he said ~50$ so I used it to "rent" (paid a friend who owns it) the 70-200/2.8L for a week (approx. the price I'd pay on borrowlenses.com). So yeah, I didn't get paid for my time but then again at my level of skill and experience my time
as a photographer isn't really worth anything. I walked away with a ton of experience and he let me file the pictures away for use in a portfolio, as well as getting to see a photograph I took printed on posters. I value those things far more then the hour I spent pre-perfomance practicing, the ninety-minutes during I used ~800shutter-clicks, or the 20miles round-trip I covered myself. Its all about perspective.
Also by doing this and letting her know you're doing this, she'd be hard pressed to pull an argument of "I paid you for something and you didn't give it to me," let her know you're spending the entire sum on expenses and I'd be amazed if she weren't shamed into silence over dissatisfied photographs, after all you did spend 3hours shooting and only you know how many hours processing - for free.
Since you were looking for willing participants for practice in the first place (not a payday), I think this may be the perspective you should adopt for this event.
Yeah that was way to late and I should have been in bed... I was looking at lenses for me to rent as well if I ever need it (I have Nikon)..
