It's not that expensive if you think about it in proportions.
Full Auto = expensive. Period. It's a simple case of inflation when purchasing the gun. They are also expensive to shoot.
Destructive devices= expensive because they are rather rare and the ammo is rare too. Finding places to shoot them is difficult as well.
DDs and FA are basically toys for rich boys.
SBRs and SBSs aren't that bad. Think about it. You can basically get an SBR AR-15 today for the same (or sometimes less) than a standard AR, due to the Obama panic. SBSs aren't really that much if you Form 1 them and make them yourself. You can buy a Norinco double barrel, Form 1 it and hack it down yourself for ~$450-500, including the $200 tax
Suppressors are a mixed bag. .22 suppressors, are relatively cheap $400-500, when including the NFA stamp. I just bought a AAC Pilot. It was ~$500 including the tax. I'm looking at 9mm suppressors next. I expect to pay about $1000 total for the can, the threaded barrel, and the tax stamp.
Another reason class 3 items cost so much is because dealers can get away with charging whatever they want. Many people, including those on THR erroneously believe that there is some sort of exotic process to owning these. There's not. The paper work is stupidly simple. Anyone who says it is not, like unscrupulous dealers, is just trying to get you to buy into the "mystique". Many times, you are also at the mercy of whatever dealers in your area have in stock. Unlike regular FFLs, many Class 3 dealers charge enormous transfer fees, like $100-200 per weapon. You'll hear boogie-man stories about the ATF: "oh! Now they can enter your bathroom while you're showering and inspect your toilet tank for hidden short-barrels and vertical grips!" and other garbage too. It's all bull.
So here's a summation of why class 3 is so expensive:
1) Rarity
2) Idiots in Congress
3) Idiots who run gun shops
4) Idiots who buy into the dictum of the above mentioned idiots.