We occasionally rebuild a customer's transmission instead of installing a reman part. When we do this, the labor cost is fixed. In our case with a forklift, there would be a rebuild kit including all of the seals, discs and bearings. A reman convertor would also installed. Any additional "hard parts" necessary would be approved, the replaced. Having said all this, the one constant on price is the labor. The tear down, rebuild would be the same since we wouldn't disassemble it part of the way till we saw a problem and stop. There would be too much risk that other worn seals/ hard parts would fail soon afterward. So a labor price, even if only replacing the seals, should be predictable. If you are thinking a half day labor, at say $90, then your labor cost would be around $360. This seems very low to me, so I would consider getting a labor estimate before you begin. Heck, R&R is the hardest part. And you are handling that. Also be sure to clean or replace any external cooler hoses and cooler to remove any contaminates.