Bob,
I have seen these sets go for as little as "free, take it away" to about $200. I recently bought 3 of them for $50. I parted them out as I needed the screens, due to stupidity. As Dave has mentioned don't clean them , just dust 'em.
The CRT's are 4" and were designed for this set, you may be thinking of the 3" CRT used in the Norelco projection assembly used by a few manufacturers, Scott, FADA, & Decca. I have a PDF file of a 1947 report on this set on my web page
http://www.myvintagetv.com/philco_48-2500.htm, if you are interested in some background on the design.
The one critical part on almost all projection sets is the condition of the spherical reflection mirror that is used. The mirror is front silvered and over the years it will have deteriorated, so the light output will have dropped off greatly. There are companies that will re-silver the mirror, about $125, but if you intend to use the set it will be the difference between night and day. I've had 6 mirrors done over the past couple years.
The flat relector mirror can be replaced with a thin rear silvered mirror with very little impact in quality.
Just about all projection sets you will find use between 22 and 25KV due to X-ray production when raised above that. When you get into commercial projectors the sky is the limit, I just finished restoring a Theater Network Television that uses 36-38KV and getting ready to start on a color projector that uses 40KV+ (VERY SCARY)
And there were sets that used 80KV
Chuck