Agree with the other girls, if an invitation is 'pay your own way' then it should be stated quite clearly in the invite, or advised straight away if its a verbal invite.

It's a hell of a lot to fork out to attend a 10yo's party

I think its perfectly fine to advise you cant go due to cost. Its a bit presumptuous of the mother to think everything should just be able to rock up and fork out the cash to be honest