I had my gutter's cleaned and they said I needed 5 ventboots replaced for $525. What is a ventboot (wasn't home when they came)? Is it necessary? Does price sound reasonable? Can't seem to find anything in Google about gutter vent boots. Thanks.

Venbt boots are not part of the gutter, they are the flashings around pipes sticking through your roof. Try this site U.S. Inspect Home Inspection: