There really is little value for portraits with regard to usage rights. Its not a commercial market, and thus really has no established pricing for usage.
I would base a price for the digital files based on your average print sales per client. Some clients will print more, some less, but it will all even out in the end. Further, your profit margin would be higher since you have no direct print costs. So its a reasonable trade off financially. I would also stipulate these prices are for personal use, the same as if they were prints, and any commercial use would be priced on an as needed basis and per current market value.
If they persist for commercial use, select a number of likely uses, figure a term of say 10 years, add them all up, and give them that price. It will be in the tens of thousands and you can negotiate down from there. I.e., you won't be doing billboards, great, I can save you $3K there. No POS posters, another $1.5 savings,... Hopefully, they will get the idea.