Oh I don't think that next summer is a topic for another thread next year at all.
It's absolutely their prerogative to treat the visit in this rather formal way which has the potential for embarassment of you all, including their son. That's what's happened: they either haven't got the nous to pick up on it (they know you are short of money FGS!) or, they don't care.
So, you should have absolutely no problem whatsoever in being equally blunt, and equally blind to the fact that you may offend. Er, why should you be taking care to pick up on little hints on what's best for MIL and what she wants to happen, when she's basically done exactly the opposite for her son's family?
I suggest you both make it very clear, with lots of bland smiles, BEFORE you leave, that you look forward to them visiting you next year. When MIL doubtless gets upset or huffy, say - sorry, but this holiday has cost us dear, we had no idea you would ask for so much money. It may not be much in real terms, but we can't afford it and wouldn't have come if we'd known you'd effectively charge half board. All we can offer from now on is our own hospitality, we can't afford to pay to give you time with your grandchildren. Just so we're straight. Bland :)
If for whatever reason you and your husband feel that you can't say that to them, then THAT is your problem - and, I suspect, a big part of the reason why you are letting this cloud your stay. I think you are worrying not only about the money this time, but are utterly narked by the fact that this is shaping up to be an ongoing problem.
Newsflash - it isn't. It doesn't need to be, at all - work on that bland smile and the 'sorry, no, that won't be possible :)' - talk to your DH, come up with a plan to make it clear to them that this won't happen again, and how you will totally ignore any attempts at emotional blackmail - 'I'm sorry, are you saying you'd expect us to get into financial trouble next year for your benefit?' ... and I bet you'll feel loads better despite the current cash situation.