Oil in radiator
 

Well, I just found out that I have oil in my radiator :(

Is that definetly the head gasket? How much would a job like that cost on a 2000 Honda Civic LX would you say?

Most likely a head gasket. Random guess of about 6 hours in labor - $100/per hour, $600 plus parts cost. You might as well replace the timing belt while the head is off unless that was just done recently.

has the car ever overheated? if so then yes it more than likely is a blown head gasket.

things may need to get machined down if they are warped in which case it will get even more expensive.