"Ah. So this is the real problem. Your assumption has led to an embarrassing situation for you, which isn't really the B&G's fault." Yes I agree
"I'd be interested to hear your son's take on it and your husbands reasons for thinking you're BU." Me too - op has been asked by several posters if these friends are actually liked by son and bride and has not answered this.
I'm the same age roughly as op (mid 40's) at my wedding 20-odd years ago (which myself and groom paid for - but personally I don't think that's relevant), we invited now ex-mil's best friend to the wedding as a courtesy, she was a flipping nightmare! Rude, bigoted and when I went to what was to be my home following the wedding to change out of wedding dress asked "what are YOU doing here?" In a 'how dare you' tone (she and other guests were sleeping in the house as they were hundreds of miles from home but despite being EXTREMELY wealthy wouldn't stump up for a b&b even).
My mother (who is generally an "oil on troubled waters type") had to be reined in by my father - usually a troublemaker - because she wanted us to invite distant relatives I'd never even met!!
People go bonkers about weddings. No they're not just the b&g day BUT it's certainly not the in-laws day either.
Op you've already asked your son, he's given his reasons, which I suspect are polite ways of saying he can't stand them! Or they've offended him in some way that you're unaware of.
You should not have assumed they'd be invited that's a huge faux pas for any event where the invitation isn't yours to give.
"He said ok, he seemed relieved." How on EARTH did you get from that the friends would definitely be invited??
And I also agree you keep contradicting yourself - if the money wasn't an issue why even mention it?
Ah - because it IS an issue but not really related to the friends thing.
Your husband offering them a "blank cheque", them planning poorly and starting with what they want THEN trying to figure out how to pay for it...
Sounds like piss poor communication all round! And 2 people (b&g) who haven't been taught the value of things, so you're kinda reaping what you've sown there.
Sounds like a meeting (you, dh, bride, groom, brides parents) need to sit down and have a proper sensible discussion about the finances.