HiTech charges a little less than $1000, the rest I know of are somewhat to a lot higher.
With one exception, not mentioned in this thread, +/- 10% efficiency was the case last I ran a comparison for a company that could afford to buy more than one.
Since what you get is pretty much the same
personal preferences and the companys support staff becomes important factors.
The 'problem' is that the '51 is not very well suited to "C" and the various implementations are trying to make the best of a bad situation.
I would gladly pay much more for a REAL '51 compiler. A compiler that amongst other things would use jump tables for switch statements, allow true inline assembly (not the stupid .src) fully utilize the double dptr in the derivatives that have it, convert
for (a=0; a <9 .. to
for (a=9; a != 0 to take advantage of the simple zero check instead of an arithmetic conversion.

Erik