Well frankly I would be appalled even if you did have the money to pay for it that they have the bloody cheek to ask you.
On th eother hand say you would be delighted to pay for what you could afford and pay for the registry office and a few rounds of sarnies at a local pub.
The MIL in question wants a red hot poker shoved up her bum for being so cross about it, agree with others get her to pay for it after all she's the mum. Hey grab one of your mum's laundry baskets and lump her one.
Can't believe how cross I am for you on this one!