What is a fair price to build a retaining wall?

I'm building a center section retaining wall that is 50ft long and 4 ft high. On either side of that it's going to step up from grade (the base of the 4ft wall) and have a 2 ft wall that runs for 50ft on each side. Then 4 - 50ft runs that are 30" high with each set have a 5 ft stairwell built into it. Also, not sure what to use as a base material. One guy wants to use limestone base but this seems pretty expensive. Is there anything cheaper for the base and what would be a fair price to pay someone to install all of this. I'm buying all material. Thanks