That's a loaded question since the largest selection commonly available to most people may not always warrant the cheapest prices. And it depends on what exactly you're looking for (and I'm assuming you mean aquatic turtles). That being said, given my knowledge on herps, I could probably go out and get everything I need for a turtle for under $60, but this would be for a simplistic and utilitarian set-up only. Something a little more fancy is obviously gonna cost more.
Although some may object, and obviously, every area is different, but PetsMart is fairly cheap as far as average retail prices are concerned. You might find stuff online for a little less, but by the time you pay for shipping, etc, you're not saving a whole lot, pus you gotta wait for it to arrive.
If you were wanting to get a nice tank for an aquatic turtle (a 20 long for starters, tho I might recommend going larger), expect to spend around $100. That would include: the tank, a UVB light, basking lamp, decent filter, a basking platform and some food.