40 bucks isn't bad to get a computer up and running. (assuming you have all the other components already). If you're going over 100 bucks, I'd recommend you not bother.
I was given a quad core AMD opteron something with a socket Q or something. Now, I don't recall if I got the socket type correctly, but I only know that after a bit of research, the only Mobos that supported the damn thing were over 300 dollars because they were workstation mobos that all could have 2 cpus installed and support like 32GB ram or something etc. (This would not include the cost I would have to incur to get the psu, ram and other components.)
I just gave mine away to the tech guy who helps our company a lot. (I got the core for free ordering something for the company). The tech guy isn't able to use it either because he doesn't have the correct mobo. xD Though he swears he is going to find the time to order the parts to "play" with this cpu.