You actually have a number of options, depending on where you are in the world. In the US generally PowerPC macs are the most available, but there are places where Power7 and 8 servers can be had for 200 bucks off a pallet. Depends on who is chucking what when.
Possibly the most interesting platform you could have access to is a hacked PS3, which is pretty useable depending on what you want to do. It would also be a cool platform to develop for. Its also really really easy to do now. But you need a fat PS3. Slims are not hackable. At that, you'll need to clean it out, repaste, and hope to god the solder material under the chiplet hasn't completely burned away. There are 32nm chips that you can use, and theres new stock 60nm chips available online, but they need water coolers to be at all competent. I want to reball a board with a fresh 60nm, liq metal, and a fat water loop.
In Europe there are Pegasos machines from 98 to 03 that can be had for around 600 USD equiv. IBM might be more common, workstation wise.
As well, routers use NXP chips. IDK what ones, you would need to research, but they exist and were at one point plentiful in the wifi N days.
Theres also SAMS440/460, and amigaOne machines.