I've done some web-fishing - Josh White 78s on Decca seem to fetch 50 cents to $2. There's a couple listed on another label (I think it's Melodynote) which are $3 and $8. Nothing found on Asch - indicates that they might be rare. I'm not posting links because the PDF files involved usually take 5-10 minutes to download, and they'll only say what I've told you. If you really want to look around, start at www.recordfinders.com .

The disc with Pete Seeger, etc. sounds like a radio plugger's special, not intended for general consumption. It also sounds like it might be an acetate, in which case you shouldn't play it except to make a recording of it. Acetates wear out very quickly - 25 plays and they sound like they were recorded in a dustbin by Colonel Sanders.

Generally, 78s are not valuable. Very old 78s (pre 1916) have a period value, 1950s rock and roll/doo wop records are valuable, good jazz and blues records are collected. There is new interest in classical records as the surface noise can now be electronically/digitally removed. Only extremely rare discs will fetch more than $10. There is one exception to all this - Kay Starr 78s usually make $30 - $50 in any playable condition. I have been told that this is because there were never any re-releases on vinyl, but I've had a CD re-release for about five years and I haven't noticed the price dropping.