This article discusses the female costume on the right. The practical Scotsman on left has his own write-up.
Bolero, underskirt, red overskirt, black and red overskirt, gloves: $60
The steel-boned corset is the most expensive piece here and not included in the above total. It could easily be replaced by a blouse or fashion corset.
The bolero was cut out of a super-ugly polyethylene-over-polyester shirt. In small doses, the material roughly mimics polished leather. (In large doses, it's magnificently terrible.) $5.
Read more: Shirt to Shrug
The gloves are from Claire's, a modern accessory store that caters to teenagers. They've got quite a steampunk theme going on right now. $5.
I bought the black velour underskirt about 15 years ago for $30. I've worn it both as parts of costumes and as everyday wear, so I could just pull it from my closet.
The red overskirt is a circle skirt created from a Goodwill tablecloth. $5
Read more: Skirting the Issue
The black and red overskirt in back was created with fabric from Jo-Ann Fabric and safety pins. The only required sewing was the hems. $20-$30.
Read more: Sewing Basics
Steel-boned corset from corsetstory.com. $80.
I created the bracer from two stretchy bracelets and a panel of pleather. One bracelet was from Goodwill ($7), while the other was at JCPenny ($10). I snapped the elastic bands and sewed each piece individually to the pleather.
Pleather is inexpensive (about $9/yrd, with each project requiring a small fraction of that) and great to work with. It doesn't fray, so you don't have to hem it. It's heavy enough to take a lot of weight, but a needle still easily goes through it (unlike leather, which can be really hard to work with).
I can't tell for sure, but I think I'm wearing one bought on clearance for $7, torn apart and reassembled with additional beads.
I don't buy a lot of stuff at Steampunk conventions. Such things are handmade and, thus, much more expensive than most of my other sources. However, the lovely double-strapped belt shown here has a built in pouch, and I got the whole thing for $40-$50. I added a larger pouch (for my gigantic cell phone) for another $20.