+1 for the .223. It is the cheapest if you don't reload, and cheapest if you do.
Any cartridge that is US Military, especially if it is being used currently, if going to be a good bet. Of the centerfire rifle cartridges adopted by the US, only the .30-40 Krag has not been a hit. 45-70, 30-06, .308 and .223 are all popular and the last two have both milsurp ammo and components available. Almost all my .223 and .308 brass is milsurp.
Even my 8x57's, 7.62x54r's and my 7.5x55 Swiss benefit from their military roots - and I have recently bought milsurp ammo in each of these calibers. I was really happy to find GP11 Ammo for my k31!