You all sound toxic!
She doesn't like you. You clearly don't like her. Why would something she says (and which, by your account. is a totally expected comment) upset you so much? Surely you're being naive...?
You keep saying it's not about the money as you were going to pay for the wedding yourselves but all you e talked about is the money. And how well off you are. The title of your AIBU is very telling.
As is:
firstly let me say that we didn't expect any contribution from ILs, genuinely. So what's the problem?
We were going to pay for it ourselves, but am I wrong to feel hurt that she said this? She has form - why are you surprised and hurt?
I feel she is jealous of my family's money... and the issue is???
it stems from this what does? Her dislike of you or her not contributing or both?
Our feelings weren't considered and never are why are you surprised? You know what she's like.
it's all about pleasing her son/DIL we cannot control how others feel towards us or who they prefer or who they want to please. Also, this sounds like something a teen would say!
The (perceived) favouritism is not your concern. If it's real, it predates you and is between your DP and his DM. No one else. If he doesn't have issue with it, that's his choice. Just know that this favouritism will filter down to any children you may have.
The whole thing seems weird - but the most bizarre part is your take on it and reaction to it.
Not getting on with ILs is a major issue that couples in love gloss over. It rarely improves - it gets worse. I have wonderful DC that have made it all worthwhile but I can assure you from experience - if I hadn't had DC and I could speak to my younger self I'd tell her - "yes, it's true love, yes he's probably your soul mate and yes, you'll have some wonderful times BUT it's not enough and you deserve better. End it now. Deal with the heartache then meet that love who, ideally, doesn't have a fucked up family and, if he does, is mature and strong enough to be in your (plural) corner.'
If ending it is not an option, the 'healthiest' solution is NC or very low. Pay for the wedding yourselves as originally planned. Have a wonderful day. On the day, treat your ILs as if they are the most perfect ILs one could wish for - honestly, it will be worth it and will make everything go as perfectly as possible. After that - minimal/no contact. What your husband to be does will be up to him.
These situations never improve - especially when both sides have deep rooted issues.