Well, that really depends on what sort of helmet it is. There are many differing opinions on DP Boba because they had different shapes depending on year. '95 and '96 STDs are considered the highly for costuming, although they're slightly under size. If it's a deluxe, I think it's pretty rare.
Anyway, I think your best bet is to ask on the TDH....
A guy in our band has the old DP basic Boba helmet, and I tried it on at band practice the other night. A bit tight and my nose was pressed against the front of it. He's going to wear it at our annual band Halloween gig, and I'd be he'll be taking it off before too long. lol
They will alll have a serial number/product number the delux will have an edition number if it's fibreglass it's a delux if it's vinyl it's a standard and really not worth anything.