No.. you don't pay until after the unit is built. You build the unit, THEN you pay for the wargear it has. You don't pay, then unpay, then build, then repay, then calculate the final value. You only calculate the final value of units and current wargear after the list is complete. This is because until the match is started the list is still incomplete. Your list isn't completely built until the person you are playing reviews it and you compare list values against the agreed list points. You don't build a list and pay for it in a vacuum. Someone else has to review your list before you actually pay the list points because you are comparing your list points to theirs if you are playing them. This is how people can add/subtract stuff from their list at the beginning of the match. If a tournament sets a point limit, the list is still not complete until a tournament person reviews and approves the list. Which is after you build the list and allocate the points for the wargear, which happens after you build the list. So you only pay for the wargear the unit/list ACTUALLY has after it's built, not what it doesn't have. or what it had before it was built.
"Simply add up the points values of all the models and weapons in your army, and make sure the total does not exceed the agreed limit for the game" pg 214 BRB