A large military base is a huge expenditure. And the wall wont do anything to defend our country in a military/public safety sense. It wont stop drugs because IIRC 95% of drugs coming across the southern border come through ports of entry.
It wont even do a lot to help the immigration problem for two reasons: first the current problem are asylum seekers and they have a right to make an asylum claim if they get on us soil, even if that soil is south of a border wall. Second, net illegal immigration from Mexico is below zero, meaning that more illegals are returning there than coming here. The illegal immigrant of today, and of the future, isnt a Meixcan border jumper. They're someone who comes here legally and overstays their Visa.
So all the wall is, basically, is a campaign prop for Trump. He promised it and he wants to deliver, even if it doesn't solve our problems