When we lived back east, we went to the CT faire every year. It was a blast! Dressed up and everything. Haven't been to many in Oregon, tho.
For cheap pants, get some black scrubs. They're baggy and tuck into boots nicely.
Also check Goodwill for shirts or vests; we've found some good stuff before. Ebay might have some good stuff, as well as Etsy.
If anyone sews, there are free patterns for simple shirts/capes on the internets, and Joanne's always has X percent off coupons on their site. The added bonus is that you don't even have to be good at sewing; it's usually a bunch of straight lines.
You can also be a muddy peasant; dark baggy clothing, and roll in a mud puddle. Instant costume!