I've read from Stevebfl and euro287 that they charge 15.5 hours labor to replace an evaporator in a W124.
Expect to pay $300-$400 for an ACM evaporator, and at $60 / hour for labor, you're talking $930 for labor.
Totals out to $1,300-$1,400.
FWIW, Hi-Power has a 1986 300E, and his a/c blows ice cold and has never suffered a failure of the evaporator or any other a/c related component. Never even needed a recharge of R-12.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".