The bet resolves as one of the options below provided the wall starts construction within the deadline. The bet resolves as the option "The wall is not built" if no decision can be reached by the deadline of this bet.

Mexico pays entirely means that they pay via any means (directly, tariffs, etc) the entire cost.

Mexico pays partially means they do pay via any means (directly, tariffs, etc) some of the cost.

This bet cancels if Trump is not the president.