As 1448 noted, tons will make a difference.  For a lot of tons, over a 10 year period of the time the bunker will easily save you money.  I don't know where the break would be, but I'd assume somewhere in the 500-1000 tons range.
A drive over pile would be another option.  Make sure you can drive over it in both directions to get a good pack. The only expense is the cover and the tractor fuel.
Bales on the side are a problem because you cannot get a good pack along the bales (they move) and so you will have greatly increased air infiltration, spoilage and shrink along the side.