If you are seeing those 21" some 19") on e-bay then don't go for them, they all have some kind of major problem. Most of them have burn in (where an image gets burned onto the screen and never goes away). They do tell you about the problems, in the fine print 😉
If you are seeing that in retail stores online then see if it has something to do with a mail in rebat or something. CRT's are getting cheaper but a 21" for 200 is getting a little questionable. A 19" for 200 would be a very good 19", you could even get a good one for a little cheaper than that. Just try to buy from a very well reviewed or well known place. Don't just buy from someone because it is the cheapest.