Brien,
There's not a good price guide for casino ashtrays. There's one by Art Anderson, but it's about 7-8 years old so a lot of the prices are waaay off.

A new price guide came out, within the last year or so, by Roxy and Gary Heller (I think). I've not seen it, but I've heard that the prices are as far off as Anderson's book!!

Click on the link below, and go to the first page of "strip" ashtrays. Go to Sahara and you'll find a hexagon that has the sphinx, and a green background. (At least I think it's on that page) That ashtray lists for $12-$15 in Art Anderson's book. I was told that it lists for $30 in Roxy & Gary's book. We got it for just under $700 on ebay, less than a year ago. That was probably a little high, but in about 8 years of collecting we've only seen 2-3. (And one of them was damaged).