I have worn contacts before to events like that haha :) Well I bought a few pairs and they average about $25 to $50 depending on where you get them. The kind i bought were by a brand called 'Geo'. To be honest, contacts are really hard because you have to get them the right size and make sure they fit your eye. Mine were too big and they kind of moved around on my eye. They were very uncomfortable and I could only bear to wear them for about two hours, and i noticed some redness in my eye after i took them off and my eyes felt sore. If you are going to get them, make sure you buy the 'soft contacts' not the 'hard contacts' because those can scratch your eye and damage your cornea.
But... there are some color contacts that are great. I wear color contacts everyday and I don't need a prescription. I have naturally brown eyes and I wanted blue eyes so now I can have them :) The ones I wear are pretty good, they're soft contacts and the brand is 'Acuvue 2'. I've been wearing them for about a year and my eyes feel great, i cant even feel the contacts once i put them in. I just forget they're there. These are however, more for people who want to wear color contacts on a regular basis. But if this is something that appeals to you then... :)
hope i helped xD