With the proper addition of some combination of things like "dungaree buttons" (in the U.S. these are not usually sold as such and you have to ask the staff for a recommendation off of the giant any-kind-of-button shelf) with braces/suspenders, a little patch pocket for my watch, and occasionally loops and little leather things to carry accessories - plus designer-label removal - God help me I have managed to make Dockers look steampunk! I saw how stereotypically frat-boy bland they are and I relish a challenge. Big hint - it's also about what they're with. A hat and a waistcoat (especially if you don't use the braces) are non-negotiable necessities to go with these.
For budgety stuff I like to select the blandest style of base clothing I can and steam them up with additions. A dress shirt, a waistcoat in a plain color sold as a "vest" from a budget outlet, the neck cloth idea (which is great and I am bookmarking that site...for fall), altered trousers, and the right hat, and you've your base outfit. If your waistcoat is patterned instead of plain color, choose the right type of print and don't make anything else on your body print unless it's an exact match to the waistcoat print.
In my experience footwear is kind of anything goes; pick a comfortable formal shoe or a fanciful boot (especially for the air pirate or the Western thing). Avoid any kind of "athletic" shoe or anything canvas with laces. I have little doubt someone, some day, will find a way to steam Converse All-Stars but it probably won't be me. However, since a lot of steampunk dressed-up activity seems to include hanging around standing up in museums, parks, clubs, conventions, and expos, comfort aces out historical accuracy in my opinion.
Most of my own clothes accessories for steampunk come in three categories: stuff I made, stuff I bought in thrifty stores, and stuff I found on the ground. I mean it. A sharp eye and a clever hand can get you a free steampunk accessory of one or another kind if you're lucky.
Since the "look" has gotten popular I have found a few more expensive ready-made things on the Internet and yeah, I've picked up one or two of these, since they're there and it won't be popular forever. But please folks. If you do want to spend more money, please don't go to the Hot Topic collection or whatever. Rather, browse something like our own trader forum here, or the right Etsy, and support a real steampunk artisan who could better use your cash. Keep it in the community is what I'm saying! If we are a community then it should count there, yes?