What is your preferred operating system? Mac is great but really pricey. I'm in the process of building a Mac desktop instead of ordering it - that will save almost a grand on the overall price.
I see a lot of good deals on Craigslist all the time, but you have to know what you are looking for before you take that step.
You can get a mid level machine and put a free OS like
Ubuntu, which will allow you to perform most everyday functions. It is even bootable from USB.