What type of setup do you have to offer? There are sooooooooooooooooo many wonderful different types of turtles it would be best to consider where you want to keep it prior to picking one up. They come in all shapes, sizes, colors and with a variety of different habitat requirements.
I've also found that the "cheap turtles" (red ear sliders, cooters etc) tend to be more expensive to care for since they require a lot more space and equipment whereas the "expensive turtles" often times require less money in the environment areas...
http://www.TurtlePimp.com is a great place to obtain turtles from as well. I know him personally and have completed several deals with him for myself as well as others.