For sure, they are a bit harder to replace than the battery in my Perdix, but $90 to change it seems like robbery. Even the "kit" that some of the DC manufacturers sell are a ripoff. A battery and o-ring for $20-$30, possibly more is ridiculous.
It's whatever they paid for the kit(*) plus the tech's hourly rate in 1 hour increments. You can do $10 if you are finding your staff in Home Despot's parking lot and paying them cash under the counter.
*) At $10 it's the battery and an o-ring from the same Home Despot where you find the tech -- not that there's anything wrong with that, it's where I get mine from, but it's certainly not the vendor "kit".