2003 Chrysler Town and Country check engine light

The engine light came on shortly after my daughter took the truck in for oil change, the mechanic said the evap system has a leak and wants to fix this but it will cost her approx. 375.00, my question is I never had a problem with this truck, first time someone else touched it other than myself, is there such a leak that costs so much? Now if I wanted to find this how would I do it myself? Any help would greatly be appreciated as to I am retired and on a very fixed income.