For me, it wasn't just the cost of having someone else do the scanning.
Doing you own scanning there are multiple adjustments you can make so the scan looks the way you want it to. Here's just one screen from my scanning software:
View attachment 730484
When you send your slides off to be scanned they'll load them into a machine that will scan them pretty much the same way for each slide. No custom adjustments for each individual slide...unless you can afford to pay someone to do that for you!
So doing it yourself you can get better results.