A 200 foot wide strip of land along a 1250 mile section would only be 32,000 acres of land. Even at $10K / acre it would only be $300M. Desert and ranch land should average much less than $10K / acre.
It depends on where the wall is built in relation to the border. If the wall is built some distance away from the border, then landowners will have property they can only access via Mexico and crossing the river. Therefore that land's value will be impacted and the owner will have to be compensated for any land that lost considerable value due to the building of the wall.