Certain mud and musk turtles would fit the bill for the small size, "simple" habitat requirements, and cost-effectiveness, but I wanted to address the last line in the first post - about the turtle needing to be really friendly.
Reptiles cannot understand nor convey affection in the sense that more familiar animals with social capabilities can. There is no equivalent of a kitten, puppy, parrot or rabbit in the turtle order. Many captive turtles do have marked personalities insofar as the "lesser" vertebrates go, but they aren't built to be played with.
Do a fair amount of reading before jumping into a purchase (good on you for going so far as to ask questions BEFORE buying!) and you will have a good idea of what is in store for a turtle keeper. These animals tend to be for looks only - most just don't handle well since they cannot percieve that it means affection, or ANYthing besides harm or an inconvenience. To cut to the chase, make sure your girlfriend knows that she is not getting a pet of the traditional sort, and that both of you know that the "short-lived" types of turtles will only be with you for three or four decades (grin).