We really like our asphalt.  HERE, we have heavy ground, so if we were gonna go the cement route, we would have to add a sand lift, or the concrete would bust up and be heaved unevenly all over the place.  

Last fall, we added another smaller asphalt pad, and the price for asphalt had doubled since the last, it was $2.00 now.  For concrete, I suppose $4.00 or more would be a good estimate, but it really depends how much rebar is used, and how thick the pad is poured.  In the perfect world, for a concrete pad, sand would be placed, and one of those concrete paver machines would place the material.  They place a much drier mix, so it is stronger, kinda like half way between precast and regular poured cement.  Feed acids will eat away at concrete, so a good mix is a necessity.  

For our asphalt, we do always place 12" of base.....8" of breaker rock, then 4" of base course.  Not free, but our pads look like new for many years.  All together, including all costs: excavation, rock hauling , leveling and compacting, and 4"  of asphalt, we came in right at $3.00/sq ft.  For our 9 foot bags, we are allowing 17 feet/bag.  We now have room for 16, 300 foot bags on asphalt.  

The fact that we don't fight mud, lose zero feed in the mud, and have a very nice place to do the bagging make it work out well.  Personally, if we do the next step, it will be piles on our asphalt rather than place bunker walls.  
This is for dairy feed.  We just finished our last bag of snaplage, and corn silage from 2013, and there was zero waste, even at the end of the bag.  

I should add, Tim in WI has shown pics of his feed pad that has lime as the top coat, and his pics look very good.  Do a search on here for feed pads, and you should find some good pics and info.