Last time I tried to find one, you could get a registered LIVE knee mortar cheaper than a deactivated one but you have to do the paperwork and tax stamp and have the ATF/FBI/DHS/NAACP/NASA know that you have it so most people don't wan to bother with it.
The accessories are where the real value is. If you are serious about them, try to find one with as many accessories as possible. The canvas covers can be a few hundred dollar by themselves but you may find that the difference in pricing between one with accessories and one without is fairly small.
I own firearms not to fight against my government, but to ensure I will not have to.