And, then, take all the test data back, verify it, turn it into to usable charts, amend the AFM, get that approved by the regulator or engineering designee, have tech pubs produce and send it to the operator with any required placards, MEL changes, AMM changes, if required, like an new inspections. An OEM has lots of overhead to allocate to these offices, too.
The AFM supplement to REDUCE to TOGW on a Global, used for operations at maybe 4 airports, cost about $18,000. Steep approach (LCY) add-on, which required some repinning of the FMS and an EEC software change was on the order of $50,000. But, lots of engineering there.