...the rental car takes you OUT of Houston...get it????

I find that Hotwire...which always includes all the fees, taxes etc....is a great alternative if you don't mind pre-paying.

I did a quick search for IAH weekday and weekend and it came out at like $37 - $42 total including taxes...not sure how they would get to that number, but I like how they neatly bundle it all and you don't have to worry about all the other things (concession fees, battery recovery, shuttle bus...etc)

At DFW, there are rental car taxes that ONLY apply to DFW airport rentals. If you go to one of the off-airport rental car locations (such as the ones in Grapevine or Irving), you will pay about $15 for the cab ride and likely save at least that much on your rental.