I'd say that's about right for one in need of being rebuilt.
Rebuilt, assuming the cost is in typical wear parts, slap labor and price of a rebuild kit on top, granted your gears, case and other hard parts are in good shape, which is usually not the scenario.
$1500 will get you into a set of new gears.
I don't think there is anything wrong with a 23 spline on a street car. We have a 23 spline behind a 426 Hemi in a '70 superbee on street tires, no problem.
You don't need 18 in an A body with 450ish HP/TQ.
