Generally we start with the total prize money divided by the number of entrants. This gives you the EV of the tournament assuming everyone has an equal chance. If the EV is more than the entry fee, then we say the event has an overlay. If this EV is more than your total expenses for attending (entry fee, travel, food, lodging, etc.), then the event may be profitable for you.
Adjusting the EV to account for your skill level vs the rest of the field is more of an art than a science, especially since, the first few times you play an event, you probably don't have enough information yet. However, if you assume that you are more skillful than the average participant, then you can probably safely conclude that your EV will be somewhat higher than the one calculated above.