How much salt is absorbed by meat during brining

I suggest starting with this Cook's Illustrated brining article for more information on brining.

Much of what would be transferred into your turkey is liquid from the osmosis, but some salt, and sugar if you choose to use it, would be added as well.

If you purchase an "enhanced" turkey that has already been injected, you don't want to brine it. The injection has a similar effect. If you've made an enhanced turkey in the past, you've already eaten a bird with added sodium.

Harold McGee states that the turkey will gain 10% of its weight in a combination of water and sodium. Your brine comes out to about 3.9% salt, so your 12 lb turkey will gain ((12 lbs * .1) * .039) = .0486 lbs = 21 grams of salt from a 3.9% brine. Your average soda has 55 milligrams of salt per serving. The total amount of salt from your brine will be spread over the entire turkey, so divide that amount by the number of servings you expect that turkey to provide to get the number of additional milligrams of sodium per serving of turkey from brining.

If at the end of the day you don't want to brine, in that same article McGee also suggests a way to keep turkey moist without a brine.

EDIT: I did my math wrong above, and have corrected the formula and results.

I think (as is so often the case with preprocessed meat) it will often depend on who you buy it from. I often buy kosher prepared birds (which are always pre-brined salted) and I've never had any complaints. The nice thing about kosher preparation, is that it is done according to very specific rules, under strict supervision. It's not a factory process.

On the other hand, I cannot abide the brined birds from major poultry companies. Their primary goal is to increase the weight of the bird, not the flavor.

Factory prep seeks to minimize time and cost. They will use cheap ingredients (largely just water and salt, with no added herbs) and they will use whatever process takes the least time, most likely injection, which has the added benefit that they can force more liquid (and therefore weight) into the meat than would normally come from absorption. They'll likely insta-freeze it, so wait time is probably not an issue, but freezing causes its own issues.

I would say, for best results, take your time. A nice leisurely soak, real salt, and no pressure on the meat. For best results (talking turkey here) look for "Natural" or "Minimally Processed": if it says that, you shouldn't have to worry about competing additives.