The small coffee bar there is the cheapest Costa Coffee in existence! They also stamp a card for you so that you get the tenth coffee free.
The photocopying cards are cheaper 20p per copy against 25p cash (A4 & A3 are the same price), but you have to buy one first and they are £5 each, and about 50p (I think) is taken straight away for the cost of the card.
I agree about the first visit being possibly disappointing, it is all quite overwhelming at first, so much to see and do, but the staff are very helpful.
One point to note, if you have any loose sheets with you when you go through the security to the reading rooms, they will staple them together so if you have anything that you don't want defaced take a copy.
Have a great day it really is a great place!
John