.338 RUM is cheaper to start off in than Lapua. I had thought that the RUM was a belted Magnum case but recently I found out that it isn't. (good for Rem-Chester, dies are probly easier to get ahold of, ammo might be bought locally if needed, more rifle choices, less money per rifle if bought stock and not custom made.)
Brass is cheap for RUM, then again Lapua doesn't offer RUM brass, yet anyways. Rifles are cheaper in RUM, they can even be bought chambered in RUM and the price still comparable to the other bolt actions. (Cheaper price due to the manufacturer not having to design a new action for the round.)
The RUM is relatively new compared to the .338 Lapua, this is probly some of the manufacturers answer as to why they don't offer the .338 Lapua. (Make money without having to spend it to retool or R&D... design a cartridge to fit the rifle and not design the rifle around the cartridge...)
I have a Lapua, which was the in-between cartridge between the .300WM and .50bmg, then that oversized .408 Chey-Tac came out.