Depends on where you are, but $40/hour and up wouldn't be unheard of.
Man, that search function is amazing:
http://forums.hybridz.org/index.php?/topic/85799-how-much-to-weld-in-new-floor-pans-and-frame-rails/page__p__819159__hl__floor%20pans__fromsearch__1entry819159