Our enforcement costs would have to be upwards of $50 billion to compensate for illegals' net social services consumption alone - and while a wall would be largely a one-time expense, our social service costs are ongoing and accelerating. Furthermore there are all the incalculable costs of importing a third-world culture, with its crime and disease, directly into our first-world cities. And of course there's no amount of money which is worth selling our sovereignty.
Unfortunately, there would not be incalculable costs in another recession - I'm certainly not talking about just enforcement costs here - what I am worried about is whether ALL "costs" from whatever happens exceeds the "savings" of $50 billion, then I don't think any conservative in his / her right mind would be for it. Similarly, if it could be proven that the "costs" were less than the "savings" I would be for the wall too.