Price depends on your area and if you are doing recognized or unrecognized events. Check out this website for your area, and you will find lots of useful information http://www.area5eventing.com
If you go to the calendar: Area5Eventing.com - Calendar and then click on the omnibus listing on the right, you'll be able to look at the prices for each event and get an idea what it's going to cost you.
In Alberta I'm paying around $350 for an event for a weekend, the lower levels will be cheaper though. It's not cheap by any means, but in my opinion is significantly cheaper than some other shows out there! It's more worth it to me to do a weekend of eventing, instead of a division or two at a jumper show.
Area5Eventing.com - Calendar