Smell of gas in the engine bay?

alright guys so my car got recently stolen, nothing serious was taken just my steering wheel, my laptop which was crappy and my oh so cool raybans. Stupid me forgot to take off the steering wheel since it was a long day for me. Got it back few days ago and today i turned on the engine and opened my hood and i happen to smell gas and i looked there's a gash that's leaking oil to the thing that is connected right before the headers... so i was wondering how much would it cost to get that fixed? or what part is connected right before the header? Please and Thanks!