The math doesn't work that way in the US,
@Kevin Peacocke . If we go to the lumber yard and get a block of pine for $10, It's $3000 of labor to fit that stock in a grotesque manner. It's another $1000 for a fore end. (these are boxlock prices, sidelocks are more) Of course you can get a straight grained piece of adequate American Black Walnut, California Claro, or Bastogne for $1000-$1500 more for the wood. If you want perfect grain flow, symmetrical pattern, and you want it in English or French Walnut, the blanks are going to start at around $1500 on up to $4000.
Any way you slice it, a restock of a double rifle done properly to handle the recoil of a large bore NE caliber, you're going to be down 2-3 years wait time on average and minimum $4000, probably more like $6500 dollars for a so-so stocking.
It is always best to deal with an extended and leather wrapped pad to make a gun servicable rather than to restock a double rifle. Otherwise, best to sell the gun on down the river to the next shorter guy and find a different gun. You cannot come out on top getting a gun restocked as an "investment" which is how I read the OP's intentions of "not losing money".
I've had but one gun restocked and it was four agonizing years wait and it was $6000. (ten year old story) But then again it was a gun I paid $6000 that was worth $35,000 restocked and was wholly worthless as a box of parts without that new stock. That is a rare scenario where the aggravation was worth the trouble. It was also a famous gun of the highest known quality with all the best features, photographed in books, and owned by a famous Nawab in princely India before partition. None of the aforementioned example would apply to a F.A.U.L. double rifle that would likely be a working man's double worth little more than the cost of a stocking.