I'm sure if one of the themeparks did a digital only pic for a quid from a self service machine they'd sell a tonne of them and make far more money from them than they so now with practiaclly no overheads after its set up.
£1 would be way too low. Take 20% off for VAT, and you're left with 80p. iMagic will take a substantial cut of that, let's say 70% (following the Apple model), leaving the theme park with 24p per photo. This is without taking card processing fees into account. If you're doing it as an In App Purchase, then Apple and Google will take 30% too. In this scenario Apple would have 24p, iMagic 39p and the theme park with 17p per photo.
The overheads after the set up, without In App Purchases, would be in the card processing (roughly 0.6% for debit cards 0.9 % for credit cards), digital storage and delivery and a staff member to operate a card terminal. We're going to be talking pennies per photo, and one would presume that iMagic are going to be taking on all of the costs (apart from card processing), but it eats into already right margins at £1 per photo. People assume that digital delivery is free, or cheaper than doing something in analogue, but that's because for the past 20 years we've been living in a bit of a false economy where companies take an initial hit to try and push new technology.
Volume would normally be the usual answer, but there does come a point where the margins are so small it's pointless, or volume is actually going to be more of a problem moving forward.
A charge of about £5 per photo, for digital, would seem about right, but the park still wouldn't be seeing much from that either; 85p per photo, but volume here would make it worthwhile.