My advice is shop at thrift stores. My petticoat, top hat, gloves, red velvet capelet, and a few other items were very cheap and bought at various second hand shops and thrift stores. Ebay and Etsy.com are also great places, I bout my husband's pocket watch off of a Hong Kong Ebay store for $10 and its really nice.
Another piece of advice is go for a basic outfit: for a lady go for a black skirt and nice button up blouse or a gent black slacks and a white shirt and ACCESSORIZE the hell out of it. Accessories are cheaper than whole outfits.
Most Steampunk on the cheap will tell you to make your own stuff. My hubs is better at this than I am so I just go for keeping an eye out for brass colored jewelry, vintage lace, and other items i can make an outfit out of rather than tinkering away and most likely hurting myself ^_^