For reference I paid $1600 for around 50 photos in the cheapest book the photographer had to offer. We could select photos from 1000 for review, out of which a lot of them were duplicates, closed eyes, weird faces and pictures of trees, pots, and vases. He was only there for the ceremony, and 30min photo shoot with us afterwards, and my wife's getting dressed, a total of 2-3 hours with driving. On top of that he was very rude most of the time, for which he later apologized.
Gearwise, he carried 5D (dont remember which mark) and 60D, one with L telephoto zoom 70-200 and one with L standard zoom.
So in terms of gear yours is inferior to the "professional" (in the lens department), but it is very difficult for you to be worse in the way you acted and the photos framing and picking the right moments to snap.
We won't get the photo digital files, so the physical book is all we will have from those 1600$ along with our bad memory of him being there. We paid before the wedding.
So the main question is what does physical book vs digital files translate into price differences? The book is an expense for the photographer and takes some time to plan, while the digital files give the customer a vast freedom. You spent more time at the wedding itself. Let's say the book incl time is half the cost.
But they do get files, what should you charge for those? $300? Price is now $1100
You are using consumer grade lenses on the same camera body as the pro. What is that worth to the customer: $200? Total price: $900
Do you have a photo store to pay mortgage and accounting on and run hat needs to be funded this way? No: subtract the 20% overhead (20% is the usual overhead given by public funding agencies to companies): $720
Are you planning to pay tax of this amount like the pro, or put it in your pocket? If no tax: $533 (assuming 35% tax)
Resulting Price: $533*
*) given some assumptions of a subjective value of some things, but I think my approach is an example of a price estimation procedure, that takes you through some points of considerations that you should take into account.
Now.. this is what you could tell up front. After the fact you are left at a disadvantage.
If you haven't given the photo files yet, you can adjust the amount you will give them based on the fraction of x/500 , or consider it an investment of your future career (if you are going for that), and ask for a symbolic amount of $250.
You could start by asking them how much they are counting on. Maybe they will just go "we were counting on 300 bucks", if you were not going to be invited anyway. If you were invited anyway, they might think it was free.