2000GRNSE

ok, guys my service engine soon light came on got it checked and it reported back a mis-fire, but the cause unknown. I can definitely feel the loss of power especially when accelerating on the interstate. Also, when shifting from 1 - 2 (auto) it's not as smooth as it use to be. I have always used premium gas. And have 76,000 miles. I've had all the recommended services. Any thoughts as to what may be causing the mis-fire and what it'll cost to fix? Thanks.