How do i tell my friends i cant afford it?

well this is actually really embarassing but a couple of friends asked if i could go to the cinema with them but my family is running low on the money. i knew that so i said i had to go out with my auntie and that. so one said go with them another time we know how much you want to see it. you see they come from very rich backgrounds and dont get it when i say i cant afford it. they just think, 'what its only going to be a tenner'. that may be but its �10 i dont have. they wont take no for an answer and i dont want to say im broke because its so embarassing! how do i get them to just leave it? thank you! :D