The cost of the stuff depends on your connections and local prices if you plan on going local. You need to look around. We can't tell you what your local prices are....
The 4T60 will most likely blow a diff housing before you would consider upgrading it to parts that are solid as Bill mentions. While Bill and I disagree about which trans will hold better...my transmission made it to 175K miles and drove 12 hours to the shop. (Not leaking). His went around 110K? and leaked the entire way. Both needed to be rebuilt.
The way I see it is GM didn't torque manage the 92-95 HD trans the way they did the 96+. Reason they TM'* them is they learned what was prematurely killing the trans was the torque. They fixed many of the known weak points in the trans (diff area, cavitation of pump etc) and didn't overbuild the rest for the torque. Instead they used software to hold back some fo the torque/power to increase the longevity of the transmission.
Now.. there are more people modding S2'* further than S1'* because of the more easily availabe parts. Once TM is dinked with and the trans is getting more power than it'* built to handle.. splat. Push an S1 too long and splat. Both can and will fail