Cost estimating is a pretty tough without specifics. The unit cost for 1,000 SF is much different than 10,000 SF. A private sector fill wall with on-site soil may be nearly half the price of a DOT wall with all granular backfill.
If $15/SF - $50/SF is close enough, then you are done.
Soil Engineering by Spangler and Handy, 4th Edition has exactly what you are looking for regarding a rock face. It is not covered in earlier editions though.
Full earth pressure usually occurs to a depth of approx. 2 x space between wall and rock face and then levels off assuming a certain...