I am in agreement with iguana_kiev about the rarity order, from most rare to least.
1) Cyrix 5x86-133GP (only ever seen 1 of these on eBay plus the 1 group buy)
2) Cyrix MII-433GP (I've actually seen two of these on eBay, but at the time, I felt $30 was too much)
3) AMD X5-160 (Never seen one of these on eBay or CPUWorld, but there seem to be quite a few reports of people using them)
4) AMD K5-PR200 (It used to be a lot easier to find these on eBay back in 1999)
My order of most desirable
1) Cyrix 5x86-133GP because it is the fastest stock socket 3 ever made.
2) Cyrix MII-433GP because it is that last real Cyrix CPU and may be more overclockable than the more common MII-400GP.
3) AMD K5-PR200. These chips don't OC well, so this little deal is the fastest of the K5 line before AMD switched architecture directions entirely. It was quite an amazing little CPU for its time.
4) Not terribly excited about an officially marked X5-160 as it is not the fasest socket 3 available, and the X5-133's with CPUID 04F4 can almost always be upclocked to 160 MHz.
I really like the prices iguana_kiev posted. That is a really good attempt to keep these CPU's priced low, however I'm not sure how realistic it is. Tetrium, would you sell your Cyrix 5x86-133GP for $120? hahaha, you, you don't have to answer that. Since I don't want this post to influence collector prices, I'll just rank them in terms of which I feel is the most cost valuable.
1) Cyrix 5x86-133GP $10
2) Cyrix MII-433 $9
3) AMD K5-PR220 $7
4) AMD X5-160 $6
I once had a Cyrix MII-400 rig at 300 MHz in a case. It was in one of those problematic FIC Super7 boards. It was significantly slower than an AMD K6-III+ at 450 MHz. I am also planning on rebuilding this rig for MII - 333 MHz (83 MHz x 4.0) operation,
Plan your life wisely, you'll be dead before you know it.