Southern shooter:
If you would like to make the comparison to AP's, just plug in $0.35 each per bullet for 50 BMG and $0.25 for 338 Lapua using cheapie Hornady Interlocks. Even if you bump the powder down to IMR-5010 for the 50 to $0.25 per round the Lapua is still cheaper to shoot, just because of the cost of 50 BMG primers.
Using the above figures, you get about $1.03 per round for 50 BMG and $0.68 for 338 Lapua.
Changing one more variable, if you use cheapy MG fired brass for your 50 and you buy it at the now super cheap price of $0.55 each, you'll only get a max of 6 firings out of it before the casehead separates for an average price per firing of around $0.092, which actually brings UP your per shot cost to around $0.009 per round for 50 BMG.
The moral of the story, you can shoot damn near anything cheaper than 50 BMG. The only exceptions that quickly come to mind are a few African Nitro cartridges, 20mm Vulcan (and other NFA rounds), 416 Barrett, and 408 Chey-tac. And bear in mind none of this even includes the higher cost of 50 BMG reloading equipment which costs a mother load more than standard equipment. Heck, 338 Lapua dies will fit in my Redding T-7 I use for 357 MAG, 44 MAG, 223, 308, 270, 30-06, 300 MAG, and the list goes on.
-David
Edgewood, NM