Nico,
A true DG double gun will cost you some bucks no matter which way you cut it.
The cheapest useable double that I know of is the S-2 Blaser. They start at about $6800 USD.
Then the Kreighoff, they start at around $8500 I think.
My rifle is a Searcy which I find to be one of the finest, most accurate, strongest built doubles on the planet plus it is an ejector rifle which the Kreighoff and the Blaser are not.
The base model Seacy sells for $9500. Go to
www.searcyent.com. and have a look.
If you want to buy an H&H or a Rigby the tag starts at around $30,000.
I reload my .470 ammo but if you were to buy it I think the cheapest is Superior for about $100.00 per twenty Federals are like $200.00 per box.
reloading I've got it down to around $1.80 a round.
Now for the realistic side of DG rifles. You can buy any number of bolt guns in adequate calibers from .375H&H up to the mighty .505 Gibbs for under a thousand dollars. I'd recomend the Winchester model 70 Safari or stainless synthetic in either a .375 H&H for the SS or .458 win mag in wood and blued. These rifles will run you right around grand.
CZ USA also builds a fine seies of mauser style rifles in their 550 line in all of the popular DG calibers .375H&H, .416Rigby, .458Lott, and starting this year .404Jefferys, and .505 Gibbs.
These rifles retail for about $750.00.
As far as ammo goes I own a .458Lott and a .375H&H I reload for both which with the availability of commercial brass is about the same price as reloading for any other center fire rifle. If you were to buy your ammo you can get .375H&H ammo for like $25.00 a box and the Lott is like $60.00.
There are any number of manufactures that I haven't mentioned who make large caliber bolt rifles for a fair price.
But if you want a reliable well built useable double break out the coin partner they ain't cheap.