The simple answer is "no."
There is not "anyone building a decent big-bore (.40+) double for less than 6 grand."
Used market: $9,000-$10,000 and up...
As discussed above by the AH community, in broad terms the market generally starts around $9,000 to $10,000 (depending on seasonality) for good quality used rifles of reputable make (Chapuis, Merkel, Verney Caron, Blaser, etc.). Heym and Kreighoff are very occasionally seen used and generally sell for $12,000 to $14,000 and up. Some very fine century-old British or Belgian rifles show up from time to time, and IF THE BUYER KNOWS WHAT TO LOOK FOR, they can be great purchases around $20,000. Age alone is not a critter for a well maintained rifle.
Closeout sales...
For the last few months, the one exception to the rule was the two new old-stock Blaser S2 .500 NE offered on closeout sale by Mad Dog Guns on Gunbroker for ~$8,500. I pointed them out a few times in a few other threads to folks asking the same question over the last few months. A quick verification shows that they are gone, which does not surprise me. They were deeply discounted and they are good rifles, albeit non-traditional.
Screen capture from November 16, 2019. It seems to be sold. No kidding! At this price it was a steal...
It is worth looking on a regular basis on Gunbroker, Guns International, etc. One never knows what will pop up...
Low cost double rifles...
There are a few threads here on Sabatti and other "low cost doubles". Likes and dislikes are opinions and generally irrelevant. However, some of these threads also provide impartial discussions of very specific technical points (I contributed some posts with pictures re. the joining of the barrels for example) and these technical points should be considered. The fundamental question is: are you buying a 100 rounds rifle, or are you buying a multi thousand rounds rifle?
Caliber? Stopper?
To each their own!
I fully acknowledge the popularity of the .40 cal double rifles. I suspect that like the .404 Jeffery bolt action rifles, they are popular because they are stepping into big bore territory, while still providing mid-bore recoil.
Are they great killers? Absolutely! Are they great "stoppers"? No.
Apparently they have always lacked the frontal area of larger calibers, the bullet weight, and the raw power to fully belong to the club. Most pros for the last hundred years - I mean the pros with literally thousands of elephants and buffalo to their individual credit - have argued that a .45+ caliber 500 gr bullet delivering 5,000 ft/lbs of energy is the right minimal recipe for a charge stopper. The 450/400 delivers a .40 caliber 400 gr bullet at 4,000 ft/lbs. It is simply not enough for a "stopper." Pretty good for a DG rifle, but not for a "stopper."
My personal take is that if someone is going to spend $10,000 to $20,000 (or more) on a double rifle, for that money one would want a dual purpose killer/stopper double. That means a .450 / .470 / .500.
"a few elephants..."
I too smile at the unhappy formulation (we all have these from time to time) and I agree with what has been said. Unless I am completely off my rockers, elephant hunts start at $30,000. A few being at least 2 or 3, this means $60,000 to $90,000. Yep, a quality double will add at least $10,000 to that, which is not negligible, but should not break the bank ...
On a budget for an elephant rifle?
Forget a shoddy double. Buy a rock-solid $1,200 CZ 550 in .458 Lott (in which you can shoot either .458 Win or .458 Lott ammo depending on how you progress in handling recoil); spend another $900 to have Wayne at AHR (American Hunting Rifles) bring it to the level of life-depending reliability of a $15,000 English Rigby; and you will be MUCH better equipped than with a $6,000 double.
That is unless you find a widow who does not know what she is selling...
