The most economical DD I can think of would be the SWD Streetsweeper. They go for about $1000-1200 for examples that are in excellent to new condition and come up for sale quite often. They are nothing more than a revolving cylinder 12 ga. shotgun and a pretty clunky one at that. Fun to play with though, and ammo is as close as your nearest Wally World.  

They were sold as normal Title 1 shotguns until the jackasses at the Brady Center petitioned the Treasury Department to classify them as DDs because they have a bore diameter greater than .50 inches and are "non-sporting". Of course the Clinton administration was happy to oblige.That happened in 1993, if I recall correctly.

if you don't want a Streetsweeper (which I kind of want one) you could buy a Spike's Havok and DD it.  There's a guy on Grog's forum that wanted to make 40mm barrels for the Havok.

Just get an M203 already.  They can be found for a little over a grand, if you're patient and lucky, and they're way cooler than a street sweeper.

I DD'ed an RV-85 and a DT1315.  Combined cost was about 50% of a street sweeper.  I'm looking for a 1325 (40mm) barrel for my 1315 to have dual caliber use.

The RV-85 and DT-1315 guns aren't DDs per se, but I'd guess Bravo is handloading stuff that ATF considers as antipersonnel rounds. If I understand things correctly, the only 37mm ammo classified as DDs by themselves are HE rounds. Antipersonnel ammo like fletchette/beehive rounds must be fired from a launcher that is registered as a DD. Kinda screwy but I think that's the way it is. I guess the same "logic" applies to 26.5mm  ammo.

I immediately thought of bored out spikes 37s

some people have.  Spike's used to make thicker barrels for just that, but stopped it.  they didn't want the liability.

Yes.  Both were originally flare/gas/smoke/signal projectors and were classified as non-firearms by the ATF.  They were both registered as DDs on form 1s.  I can now possess and use antipersonnel ammo in them, such as batons, buck shot, solid slugs, etc.  I can also use chalk marking rounds without worrying about the potential gray area in the law with these (I am extra cautious with this kind of stuff.  I am not suggesting that chalk rounds are AP.  I'd rather just not worry about some agent trying to say they are.).  I can also lawfully get a 40mm rifled barrel for my 37mm registered launcher and use any of the common 40mm rounds.  I can also potentially use sub caliber adapters to fire shotgun shells, etc, or behive type rounds that shoot multiple small caliber rounds at once.

OP:  for about $600, you could buy a de-milled RPG-2, and file a Form-1 to reactivate it.

Unlike the newer RPG-7, the older 2 uses simple black powder as the propellant and it does not use the more complicated 2-stage motor/booster of the 7.  There are also dummy "war heads" out there to use as projectiles.  You will need to repair the hole in the original tube, and reinforce it thoroughly for safety reasons.

You could do the same (form 1) on one of the 84mm Carl Gustav recoilless rifles; there are several of these in the USA on Form 1s, and a few pre-1968s out there too.  But the munitions are a separate problem that has not yet been overcome.

About 8 years ago I got a 9" Spikes HAVOC 37mm launcher and immediately turned it into a DD on a Form1.  I make my own rounds but I won't get into in on the internet.  Suffice it to say if you keep your powder charge (lift charge and or explosive charge combined) under a certain limit it is "legal" according to BATFE.  One of my favorite 37mm round is filled with a bunch of Flaschettes.

I purchased a second barrel from Spikes and had it bored out to 40mm (no rifling) so that I could use Chalk Rounds and BeeHive Rounds.  The 37mm barrel gets use with my home made rounds.  It was lots of fun to use when I lived in Utah because nothing burns at the Salt Lake.  Now that I live in PA I haven't found an area yet where I can use it and not worry about fires from my pyrotechnic rounds.