Whether Wb are undercosted isn't a historical question anyway. And Caesar, if memory serves, didn't usually fight at the sort of numerical disadvantage an equal points DBMM game of Romans v. Gauls implies.
From a game perspective, I think they're if anything overcosted. They never seem to be very successful in the games I see, even when they get to fight HI rather than get bogged down by lights or ridden down by mounted.