Hi LiqHito,
It should be pretty easy to tell if you have a Don Post Deluxe. Is it a large and heavy cast from thick fiberglass/resin? It should also have yellow foam padding and a three part elastic strapping system on the back that buckles with two silver metal pieces. Or is it large and lightweight made from slightly flexible plastic? If it's the latter it's probably a Don Post Classic action helmet. These differ obviously from the smaller "Standard" Don Post Vader helmet -- they made tens of thousands of the standard helmets at the very least and they aren't worth more that $25-$50 (unless you have a minty first issue version which might be worth 2-3 times that much).
I wrote up an answer to a similar question on another forum and here are my thoughts. I'm a fan of the DP Deluxe and have been trying to catalog all of the DPD helmets that I can and I'd love to see some photos of your helmet if you don't mind sharing. For whatever they're worth here are my thoughts about market value for the Deluxe.
I've been watching them on eBay for years now and, as much as that can be a gauge, I'd say the helmet is currently worth between $350-$600. Neighborhood of $600+ if it's a nice, minty, low number helmet with minimal distortion, otherwise they typically sell for about $500 shipped on the high end. I've seen them sell outside of these prices but I think that's a reasonable average. I paid $300 about three years ago for a damaged helmet, #79, that I am currently restoring.
The problem affecting the value now is that there is a glut of Vader helmets of much greater accuracy on the market. Even with those other helmets in the mix, for multiple reasons, the Deluxe has never held the cache, as far as I can tell, that I really think it should. I think the number of badly distorted castings they made is a major contributing factor toward this. Even when there were fewer options for a Darth Vader helmet the DPD never sold for more than $800 -$900 or so on the high end. I've seen them listed for $1200 - $1500 but I can't say I've ever seen one actually sell for that much (though it wouldn't surprise me if the odd one has).
Hope that helps!
Dave