The problem, as other have tried to point out, is that every simple attempt made to decline something ridiculous is interpreted as just not having money.
Other people interpret "that is too expensive" as "I don't have the money."
Other people interpret "that is not in my budget" as "I don't have the money."
Other people interpret "we can't afford that" as "I don't have the money."
Other people interpret "let's enjoy this cheaper alternative" as "I don't have the money."
Other people interpret "no, thanks" as "I don't have the money!"
So, pretty much the only this we can do, I think after reflecting upon this post, is use the yes/yes/no/yes strategy, my "go to" for declining invitations.
Say (or write) something like:
"Listen, thank you for the invitation. What you are suggesting sure sounds like fun! I wish I could go and I have enough money to cover this activity, but right now I'm trying to avoid being up to my eyeballs in debt like the average American. I need to invest that money now so I can retire comfortably/buy a house/pay for college someday/whatever. I really want to see you, but I'm going to have to pass on this particular trip/dinner/whatever as it is going to disrupt my carefully thought out savings plan. How about we do [insert cheaper but still fun activity] together?"
Remember, the pattern is yes yes no yes: Thank you, sounds fun, I'm not going, let's do something else or please remember me again next time you go.
Wow, that is a lot of work! It's much easier to do in writing, like via email, where one is not having to argue and defend one's sensible spending. But I really think that's the only way we, as a group, can avoid coming off like the couple in the OP. Some people are still going to think we're cheap douchebags for not joining in all of their spending sprees, but at at least we're being polite and making it clear where things really stand.
Good luck with it, everyone!