All of the websites have access to the exact same packages and pricing, they are the same down to the penny. I used Red-Tag to do my homework simply because I liked their interface and how they make it easy to see the included taxes:
Vacation Packages
OTOH, $500 taxes in is not much for a 1 week in Mexico, even though prices are much cheaper now than they would be a few weeks ago. I don't know if you'll be able to find anything at all... You may want to be a little flexible with the budget as you may end up finding that you really can get a much nicer place for only a few hundred bucks. Also, I'd look at Cuba and Dominican, while Mexico is a bit nicer, these places are also considerably cheaper and have way less taxes on the trip, which adds up. The typical Mexico trip has around $300-$350 per person in taxes and most of the Cuba vacations are sub $100.
Mark