I don't think I'm telling you anything that you don't already know, but I recommend looking in Ebay if you're going to look online....and stick with standard PC design rather than an exotic and uncommon "microclient" design.
Taking a quick glimpse through Ebay, I see standard towers that go as low as $116 w/shipping (and maybe lower...I didn't look any deeper), but less than the $150 for the cheapest microclient.
If size is your concern, search for small-form factor PCs. Some Ebay sellers sell them in bulk.
I'm thinking 2-3 computers, I'm looking at setting my lab up with some stationary pc's for running end to end ping tests. I may set one as a server client and host services off it like DHCP FTP TFTP and other things that can be useful in a lab. I just didn't want to make my main computers part of the lab because I use those on a day to day basis. I'm not trying to run games on them or any hardware intensive applications, I would however like to be able to use them for lab training.
I know about ebay but I've been looking at other options ebay has a few nice machines with operating systems already on them but thats more of the luck of the draw I'm just trying to increase my options. One thing about looking for something on ebay however is it takes a lot of search modifiers to get the right results an example after going into the pc desktop section and selecting buy it now only i still have to add about 25+ modifiers so I don't just get junk: