for paid trips, depends on fare rules. i've done several europe-us trips where i first go to conference location in the u.s. for a week then head home for a week (lax) then go back to europe and stopover costs ~$100 + taxes/fuel charges on top of original flight cost (middle of summer when availability on flights are low, stopovers allowed on each end but not the cheapest fare class). sometimes it's cheaper to buy it all separately (winter/spring/fall when availability on flights are high, cheapest fare class available and no stopovers allowed).