You could look at it this way...
If you give/sell them the digital file...they will make prints from that, rather than buying them from you. So you should charge as much for the file, as you would have expected to make in print sales.
Now, technically speaking...just because you give them the file, it does not give them the right to print or copy it...but realistically, how would you ever enforce that? So if you do sell them the file, you might as well sell the the rights to make their own prints and charge them for that right.
If they want low res files, that may be a different question...because they shouldn't be able to make nice prints from low res files. One problem with that, is that many people don't understand that. They may go ahead and make prints...the prints will look like crap...and the blame will fall back onto you. Even if you do give them high res files, you are not assured that they will prints them well.
On top of this, you have to consider that making copies of prints isn't hard these days. You could sell them a print and they could scan it and make their own reprints. This is not legal but they either don't know or don't care. This is one reason why I like to just sell the digital files and I usually include good quality 4x6 prints so that they know what a good quality print will look like. That's the end of the job...and I don't have to wait for them to order reprints etc.