How much will it cost to change an automatic transmission to a manual for a 1993 300zx?

you might as well buy a manual 300z unless you know somebody who will do the labor for you. There is a lot of labor involved, you have to drill a hole for the clutch in the firewall, buy a clutch assembly and transmission, you would probably also needa new drive shaft because the transmissions vary in length depending on where you get it from, and possibly still have to modify the new drive shaft for the swing bearing cushion to bolt up in the proper place. it is a real pain, but it can be done with enough time and money.