If you're looking for a 2006/2007 (which are still totally usable today given you beef the specs up a little, you just can't upgrade past 10.11), those are a dime a dozen, nobody wants them anymore and you can find them on craigslist, offerup and facebook marketplace for under a hundred bucks. I have 4 of them actually, lol. First one was $70 and the other 3 were $25 a piece (needed some work). Don't go on Ebay because it's all people wanting $150-$200 for some ewaste-tier machine, plus shipping.
2008's, you can usually find them for cheap on Ebay for some reason. I think a lot of sellers see the "A1186" on the case (same model number the 06's/07's, but they are different beasts) and they think they're dealing with a worthless computer. 2008's are cool, you can modernize them with a little bit of patching and they won't break the bank. For that reason, there are a lot of Mac Pro flippers on Craigslist and Offerup wanting an arm and a leg for 2008s that have been spec'd out with cheap server parts. Avoid those. Not because of the cheap server parts, but because you can do it yourself for less money, lol
2009s-2012s are pretty sought after and you're less likely to find some boomer throwing one away, but you can sometimes find a young kid who's just selling theirs because they need money. Check craigslist, offerup and letgo periodically. The ones on there will mostly be $1000+ fully spec'd out rigs for audio and stuff, but you can occasionally find one for like $400-$500. The cheaper ones on ebay will nearly always be the single CPU models, which are meh. You can also check government ewaste auction sites. Another thing you can do for 4,1's/5,1's is this: a lot of Ebay sellers who sell Mac Pro parts and memory upgrade kits are actually large ewaste recycling companies. They receive pallets of broken Macs and they break them down and sell off all the working parts. Sometimes they'll get one that's good, and a lot of the time those just wind up sitting in the warehouse because they're too busy to test it/are reluctant to list it as a working computer. Find a seller with a lot of feedback (like in the thousands), and send them a message asking what they currently have