Pretty much everyone here is incorrect... mines give LEVEL x 10 XP (for regular mines, so the ones you will find on Peragus, harder mines give LEVEL x 25 XP). So if you got 60, you were level 6. Demolition skill doesn't change that, but as you notice leveling does. Unlike enemies (who are level locked once you enter) mines use the current level you are. As you may notice this can get you some ridiculous XP's on later levels, and indeed certain very mine-heavy areas will grant you a level by themselves, if not more... Mephiles is right that difficulty affects skill requirements (for lock picking, disarming etc., not conversation checks).