I had the same problem as you!! I bought a 2007 double cab prerunner that did not have keyless entry. I did have dealer put after market keyless on it. However I would not reccommend going that route.
Later on I decided that I wanted remote start. I went to the place the dealer had outsourced my keyless entry job to. It turns out that the remote start system comes with keyless entry anyway and has a further range.
The brand they put on was audiovox and says Pursuit on the keychain. The price breakdown for me was : the part itself was $100
the Toyota specific part was $50.00
Labor was $200.00 ( worth it !! )
So the total was about $350 plus tax on the parts.
The remote start is the best option I have ever had on a vehicle.. Truck stays locked while running, dies if someone hits the brake, raises the hood, or revs the engine and shuts off after 20 minutes if u forget, or you can kill it with the remote. I guess I am going on and on. I hope this helps...