...but paying more than a quid or so for a Jpeg so you can print it yourself is taking the monkey.
If I were a commercial digital photographer, taking "event" photos, my thinking might be:
No-one will make proper use of thumbnails or watermarked lo-res pics so they are safe in the public domain.
Ideally I want customers to buy more than one copy of an actual paper print, perhaps one for themselves and others for family/friends (my family/friends would be distinctly unimpressed with a photo of me on a bike, but that's neither here nor there...). Prints have production overheads, so you can justifiably charge more.
High-res JPEGs have lower overheads - a bit of post-processing, RAW->JPEG, cropping, colour balance etc, but no postage or printing/paper/ink needed. However, no-one will buy multiple copies of a JPEG - once it's released to the customer, the photographer has no control over further distribution. Customer is free to email it to doting Aunties, print it themselves many times, what have you. There may be CD&P Act issues but who could be arsed chasing that up? I reckon you'd have to be pragmatic by charging "market value" ie as much as you can get away with and let the customer do what they want.