My actual experience with making commercial design plugins for Photoshop and Sketch (http://designplugins.com/):
1. Exporting image assets from Photoshop, <$100 in 6 months, >1 year of work on and off
2. Optical adjustments in Sketch, <$1000 in 2 months, ~1 week of work
3. Exporting ICNS/ICO from Sketch, <$50 in 1 month, ~1 week of work