1997 Chevy Lumina How much to repair a leaking head gasket?

My Sister bought her car new from a dealer. My main question from her is How much will it cost from a local garage on average to fix? The damage is just beginning new. Small, Trickle slow leak is begun to show top center of engine block. Can be observed only by stand from driver side of car with a flashlight. This is a 3.1 liter v-6 engine. We already Yesterday had a shop install a brand new radiator. Too bad that wasnt all the problem was. So least price amount to Worst?
How much do you think this will cost? Hypathetically, If the heads can be Re-surfaced/, just installing new Head gasket. We do not anticipate needing to replace the heads as the car never severely overheated. The car was shut down to allow self cooling. Less than $500.00? We hope?