Prices do not include tax stamp, to my knowledge. I've never bought one. But, I'm quite sure that it doesn't include the tax stamp. Getting a suppressor is expensive!
Why are they expensive? Because they can be. Pretty much.
IF / when suppressors are taken off the NFA list, and manufacturers start cranking them out in large quantities, you will see prices plummet. Until then, they can charge what the market will bear.