Don't put much faith into what a fast lube facility tells you; at all.
If the fluid has never been changed then yes it should be changed now. Not ever changing the fluid is usually what causes the transmission to fail; not changing it after the fact.
If you're losing fluid then it's due to an external transmission leak, a faulty vacuum modulator, or a leaking transmission cooler. No matter which one is behind this is needs to be repaired promptly as an automatic transmission can be damaged very very quickly due to a low fluid level.
As to price, the most serious cause of a leak is the torque converter seal, which requires removal of the transmission. After that the next most costly repair is a leaking trans fluid cooler and this requires replacement of the radiator or the addition of an add-on fluid cooler. My preference would be the add-on cooler as it's cheaper and works far better than a radiator unit.
Other leaks are generally not major at all.