I've just replaced 18m of iron bark sleepers with concrete sleepers on one side of my block, the other side is getting replaced in the new year. 45 sleepers, tinted with a charcoal colour, came in at just under $40/sleeper, so a little over $2K.
Depending on how high your existing retaining wall is (mine is 1 metre high on both sides of the block), it may be easier to replace your existing sleepers. My I-beams are in the ground 1 metre deep with a 1/4 tonne of concrete each beam.
The very thought of getting them out of the ground to replace them with any other sort of retaining wall convinced me to use the existing i-beams and just replace the sleepers. To be honest, I didn't do the hard yakka, I paid my son and his business partner to do it. He thought he could get it done in a couple of weekends, he's a fit 27 yo, and it ended up taking him about 6 full days.
I had a bit of a giggle when he said he could get it done so quickly, but at least he can give a quote on the labour required for doing a retaining wall replacement now. I think he learned a valuable lesson.
It's a bugger of a job.