the two places I know that make quality featherbed frames are unity-equippe and Dresda, both in the UK. Dresda does not list prices but his frames are more for specials where as the unity equippe frames are more for resotration and manx replicas. These days there is no such thing as a cheap featherbed although used ones are still less expensive than new on initial purchase.
I think barlow can get them also, I haven't checked in a while.
I guess "What is cheap?" and "what kind of featherbed you want?" and "what do you want to do to it?" should be answered before we go any further.
Just so you know pinche, there are different types of featherbed frames, most fall into wideline and slimline category but they really depend on the bike. Atlas based widelines can be had used for between $500 and $1200 depending on how bad the bike is, Slimlines are a different story since they are earlier and less common.
The only rule of thumb I learned was that if you want to stuff a big motor in it (vincent, 650cc triumph, honda-4, HD, etc...) you want a wideline frame. If you are doing a single or some smaller motor (usually 500c or smaller) you want a slimline. Of course there are exceptions to these rules also and really I am just repeating what was passed on to me, with enough skill you can make anything fit anything.
Randy Illg at Framecrafters will build you a featherbed frame and motor mount plates, too. He is not far from yo either. Just over the IL border center of the state. However, and furthermore, it won't be cheap. I would expect to pay over $2000 for a bare frame from him. Check out his site at framecrafters.net. Randy is a good guy and supports vintage racing. He has built many frames and I know he has featherbed experience, because I used to ride his Triton at Grattan in the 80's
It was more of a curiosity type of thing. I'd shoot for a wideline since I'd want to drop a 883 into it. It's more wishful thinking then anything as I got enough on my plate right now three bikes and a one car garage and a limited income makes project progression at a snail's pace. I looked around at framecrafters.net and frame and swingarms started at 2000 and up. I'll just have to keep looking around CL as I saw one pop up a few months ago but didn't have the funds then. I guess cheap would be around the 500'ish range. Ehh, someday...someday.