I couldn’t tell you what you should charge but most of the good portrait shooters I know don’t sell digital files for obvious reasons. It puts them right out of the print aspect and rather than go to a pro print lab the client will invariably have their prints done at a Wal-Mart or the like as a further insult. Then when grandmas, grandpas, aunts, uncles, friends, dogs and cats want prints they don’t have to come through the photographer to get them. This is part of a successful business plan. If, in the rare circumstance they do sell digital files, they are priced with this in mind; i.e. quite expensive.
As an aside this is also a major reason most of today’s fauxtographers will fail. They don’t have a business plan that could possibly succeed.
But having said that (and I wasn’t referring to you or your situation when I did) I presume you would be providing low res files to your client. And presumably you would have the usage limitations spelled out in your contract. This is what I would base my pricing on if I were in your position.
More usage permitted, the higher the pricing. And I would probably be looking at a minimum of my regular print pricing (again depending on usage terms).
If prints are a large part of your margin, than I certainly wouldn’t want to lose that margin to a digital file.