Wow, your fast… i can be too if windows were clean-ish, but i always end up using steel/bronze wool.
For me, cookie cutter homes windows are usually 2 panes each Windows. So you have 15 windows= 30 panes on avg? Id be around 240 because they are never that clean initially. Id then would give 15% off
or more, maybe $190 if 2x a year.