I think $150+ is a decent starting point for 1st print Petal Throne. I got mine right at $150 a couple years ago. It was a Buy it Now option off eBay. I don't follow them though, so someone may have a better understanding of how the trend has been going and of course at an auction it could go for lots more or less.

I don't believe the language books came with the box set and are a separate release (I just found this out myself). I was looking for a pronunciation guide and was linked here.

The Tsolyani Language - M.A.R Barker's World of Tekumel | DriveThruRPG.com

I would be interested in those language guides but selling as a set will probably bring in more potential buyers.

I would be interested in the Blue Camel from the Theives world box by itself if you would want to sell it alone.  Already have the rest of the set.  Going to be difficult to value it as I can only find one guy selling it (and has been for last 4 months at least) for a crazy price.  One went at auction late last year for $30 on ebay, a few weeks before I started looking....never did have good timing!!

PM me if interested and we can work out something.

Thanks
Chris