What kind of car? And do you plan on doing the install yourself? If not, going by "cheapest" is a bad idea. A hackjob install can cause all sorts of problems and introduce gremlins into the system. And you don't want to find yourself in a situation where electrical problems can't be fixed by the dealer or a good mechanic without dealing with whatever boogerwork was done to slap in the aftermarket keyless entry. The best option is to look and see if it was offered as an option for your vehicle from the factory. If so, see if you can get that as an addon. I'm sure there are great shops out there that can do a great job. The key is finding one and when it comes to wiring and installs, it takes skill and experience. Typically, you get what you pay for.