I'd estimate anywhere between $65 to $80. Assuming you have an extra Prometheus body (or bodies) handy, you still need to buy the proper uniform (which seperated by Toy Anxiety, will likely include the pants, shirt, and tactical vest). Those boots look to be unique as well. They look to be one piece, but I'm not certain. That's it for a bare bones Viper. Now, if you wanted to add the backpack, grenades, rope, knife, etc., it'll cost you more.
The only things that I see that can be shared from the Vipers you ordered are probably the patches, and you can give your bashed Viper the Masada or the old school made up assault rifle. Additionally, if you ordered EX Vipers, you don't have to worry about getting additional head sculpts.
The more experienced bashers and customizers could probably say more on this matter. The only figures I've bashed are two extra Red Ninjas, and that was fairly simple because:
A) The Red Ninja EX came with a crapload of stuff, to include weapons, extra set of feet, and the extra head sculpt;
B) I already had a spare body lying around after I ported Snake Eyes to a TTA.
So all I needed to purchase for a second Red Ninja were the pants, shirt, vest, and forearm and shin bracers. Of course, I also picked up a headwrap, rope belt, black belt, tanto knife, hat, shuriken and kunia for the full effect.
On my second Red Ninja, I purchased all the above, plus an extra Pro body (luckily I could use the second face plated head sculpt from the Cobra NInja Viper), and am sharing weapons between the Red Ninja and the Cobra Ninja Viper.
You may save $10 to $15, but in my case, I'd probably order additional Vipers to get everything, including the display box.