How often should I have my bike checked out? My dealer said once a month. If that's the case, how much should I expect to pay?
Normally each bike has different maintenance schedules. You can find info about recommended schedules in the user manual of your bike.
Keep in mind that depending on the type of bike you can have very different maintenance. For example, the maintenance required on a 125ccm 4 strokes bike is very low and could be done on your own, while maintenance on a racing enduro bike can be much more frequent and harder!
As for the costs, I can Tell you a scheduled check in my country (italy) ranges from 70 to 200 euros maximum, depending on the bike and if you go to a specialized/authorized dealer (usually more expansive). Other maintenance (for example chain cleaning, brake pads etc) depends on the bike and how you ride and your actual effort in maintaining the bike.