They don’t need to cave, they’ll pay thru various fees levied on Mexicans in the US. Fees on green cards, money wire transfers, trucking fees, importation of goods etc. And if they don’t like it they can leave...or be given the boot. It’s a win-win for us.
I imagine Mexico would match the fees on people and goods entering there.
And if they don't like it they can leave...or be given the boot.
Regardless of who funds the wall Trump has said he'll be rounding them up and heading them out. So their leaving is pretty much a given.
And we can reduce foreign assistance (ESF, IMET, FMS, etc) which effectively makes them pay for it.