Hang in there OP- it really does get better! My DS was a nightmare at 11-12- making arrangements to do things with friends which just weren't possible, saying we would go and pick up friends from places which were miles away etc, inviting people over for and sleepovers on weekends when we weren't free and then having a major melt down when we had to ring all the parents and explain it wasn't happening!
It is just the first sign of him trying to be independent and strike out to organise things on his own and he will have no real idea about how much money is involved. With my DS I tried to show him things in terms he could relate to and see why things were so much money eg if I spent 10 lots of £5 plus popcorn etc taking people to the cinema thats going to be about £100- more than it costs for a whole weeks food shopping for the whole family. Or at the time that was like 5 months of his pocket money!
If he doesn't already have pocket money start to give him some and let him have the responsibility of spending it- he will make some mistakes ie wish he hadn't spent it all on day 1, buy things that are poor value etc but that is how he will learn.
Focus on some of the positives- tell him how impressed you are he has 10 friends who all want to do something with him and that it is good that he really wants to organise some things with his friends and suggest ways in which it might work eg he can take 3 friends if its clear everyone is paying for their tickets, but you will give them the lift there and maybe buy the popcorn.
If it helps, I made a whole load of mistakes along the way, but now he is 14.5 he has really turned a corner, always checks out arrangements first and the number of strops has decreased to hardly any, they are short lived and he is just starting to see them for what they are and comes and says sorry afterwards.
Good luck!