Ballpark cost of a retaining wall vs lots of lava rock and landscaping fabric?

I have a hillside in the back of my yard. The previous owner set up a little garden of mulch and shrubs. It has been neglected and is now a 100'x50' area of weeds with a rotten 100' railroad tie retaining wall at the bottom of the hill. I was thinking of laying landscape fabric in the area and filling it up with lava rock and turing it into a 100'x 50' rock garden. My father suggested looking into the possibility of doing 2 retaining walls in a step sequence to make the yard more level in that area. 1 block wall to replace the railroad ties and another about 10' above it to level off the yard at the top of the hill. I figured lava rock and fabric would probably run me about $6,000 to do that 100'x50' area I was wondering if the retaining walls and excavation would be in that cost range? I live in the Pittsburgh area.