I agree with this... fight fire with fire. You could really have some fun with this.
"That's a nice TV, FIL, are you sure you can afford it? People don't really need TVs nowaday, there's nothing good on anyway, I think you should sell it."
"You and MIL should really stop going for coffee and lunch out. It's a waste of money... you're wasting your children's inheritances".
"We haven't bought you anything for Christmas because you have too many presents already. We actually think you should consider donating some of them".
"We're economising on pens and paper so we got the kids to draw you a virtual birthday card on the tablet this year, rather than making you one. We've emailed it to you.
"I really don't think you should have turkey for Christmas. Such a waste of money that could be used for better things. I'll send you a lentil roast recipe instead."
"Why have you put the heating on? It's only November."
"You don't need a teabag per cup. Don't chuck them away, that's a waste of money, save them and reuse them. And no one needs more than two biscuits."