From what you posted, there are 2 possible things you’re talking about, and neither are a shift kit.
1- Rebuild kits. These range from $470-$550, and include parts from a total transmission rebuild (gaskets, seals, clutch packs, etc). These are eventually what you would have to go with if you’re building high power and using it frequently.
2- A valve body service for $575. Let me quote directly:
The fact is that there are no hi performance shift kits for most Nissan transmissions! However, we are able to modify your existing valve body for high performance operation.
This is a valve body modification, which is why it costs so much. Someone is working on it at the tune of $50-100/hour which makes the price high.
I’m going to hunt around a little and see if I can find an actual shift kit, but if you want a quick 240sx with an auto, perhaps its time for a V8 swap :hsugh: . I’m not sure how much abuse the Nissan autos can take. I imagine if you sunk some money into them they wouldn’t have any problem. Just get a cooler and a high stall converter.