They are the ones paying import taxes
No. The US citizens won't pay. If the Mexican goods get more expensive, then they will import goods from Canada or Brazil. Already the Canadian Tar Sands producers are salivating at the idea of having an absolute monopoly in the US, at the expense of the Mexican Maya crude.
Which means the Mexicans won't also pay if US won't buys from them then there the wall will still be unconstructed and telling Canadians to pay for it will the the cost of the goods will increase and back to where we started from in addition to Mexico now leaving the border free for movement which is now a serious concern for Trump. If USA is really serious it should be more of partnership and not command or he will threaten them will military force like he did with IRAN?.