If you don't want anything elaborate, Hobby Lobby will be the cheapest, and they do a good job. You can pick a premade frame, and then you can pick the color of the matting around the diploma. Custom framing will of course cost more.
Check out New Art on Harwin. Great work product and very, very inexpensive. Have been taking stuff to them for over 10 years and have been happy with their work every time.