50% seems like a bare minimum to me. I get about 50% savings on 45 acp. But my cost to reload 45 acp, 38 spl, 357 and 44 magnum are almost the same. 44 magnum is maybe double the price of 45 acp. So, do the math
Rifle calibers tend to be even better. Price out some mid-range 30-06. Compare to $0.50 / round reloads. By the time you're up in the big magnums the savings is crazy.
Edit : didn't see the part about surplus. Yeah, I think it's going to be difficult to beat pure surplus-grade ammo like Brown Bear. Even if you're slightly winning by reloading, the amount of time and effort involved is going to outweigh the value. For "normal" factory ammo in non-exotic calibers, you're going to do at least 50%. And when it comes to Super Atomic Deer Killer TripleX DeathPoint ammo, reloading will be a huge winner.
Also, even plain dead simple reloads are going to perform as well or better than the most premium of commercial ammo, especially when it comes to rifles. It's astonishing how good the difference is.