The 68forums is the best source available for researching the 6.8 SPC. As for specs, I'll make this simple...there are the SAAMI I specs and SPC II specs. The SPC II specs are currently the latest and greatest. To put it in perspective, I'll explain it like it was explained to me.
There are two types of factory 6.8mm ammo: Standard factory and combat/tactical loads. Both can be fired through a 6.8 SPC with the SPC II specs. Only the standard factory loads can be fired through a 6.8 SPC with the SAAMI I specs. The reason is because of the bore and barrel twist can't support the increased pressure of the combat/tactical loads. As for the SAAMI I specs, the same can also be said for the hotter hand loads.
A perfect example to compare the 2 specs is with the .223 and 5.56 ARs. You can repeatedly safely shoot .223 and 5.56 ammo in a AR chambered for 5.56. But you can only safely shoot .223 ammo in an AR chambered for .223.
As for ammo costs, yes factory ammo is expensive. The cheapest I've found is Remington UMC 115gr FMJ rounds...which is only good for plinking in my experience. The next best ammo for plinking and hunting, which I've found extremely accurate is Silver State Armory 110gr Sierra Pro-hunters. So depending on what you get it'll cost you about $0.80-1 per round. Luckily, reloading makes it a whole lot cheaper, right around $0.35 per round with the right components.
I wanted an AR for hunting with, and my choices back at the time was a 6.8 SPC AR15 or a .308 AR10. As most know, an AR10 is noticeably heavier and you are pretty much stuck with the AR10 platform only. Since the rifle was going to be my hunting rig, I took into account the weight and went with the AR15...and since it's more versatile than the AR10, I'm now getting a 5.56 upper, with the option to also get uppers in .450 Bushmaster, .458 SOCOM, 7.62x39, etc...
As for hunting, the 6.8mm SPC round will take out any medium sized game at 200-300 yds. I've also heard that hunters have taken elk and bear with a 6.8. I've yet to regret getting a 6.8 SPC.