In searching EBay completed auctions it appears that PD150's with extra batteries and a few extras are selling for about $1800 and stand-alone units are selling for around $1300.
eBay is your friend when it comes to valuation. It is quite accurate, and with something like the PD150, where there should be lots of sales, you can get a good idea of what matters (extras, hours of use, conditions, etc.). Make sure to search the completed auctions. This will give you LOTS more hits, and the numbers you see will be the actual selling prices, not just current bids (which don't mean a thing when it comes to judging value).
More specifically, look at the completed auctions *with bids*. There's a few in the list with no bids - the starting bid was probably too high. Also, you never know if the "buy it now" was bought or expired.