From your suggestions, I assume that American Folk is an acceptable category.

About the only thing I've got in my 'acquisitions list' that's reasonably recent (should be still in print), pretty good, and under $30 is:

Logsdon's The Whorehouse Bells Were Ringing (and other songs cowboys sang) from University of Illinois Press, $15.95 about 4-5 years ago. I got my copy from Barnes & Noble, to my special order. It's been out of print, but reprinted frequently so there's a good chance of finding it.

If they don't already have it, the Warner Traditional American Folk Songs, Syracuse University Press, is a treasure of "lore" - and a valuable counter to the Lomax collections. It was in print recently, and should run about $30, if still available.

If they're into 'merican folk, you might be able to still find:
War Songs & Poems of the Confederacy by Wharton,        Castle Books, that I picked up from Barnes fairly recently - but I got it off the "sale rack" for $10, so they may have been dumping remaining copies. Some interesting lyrics, no music, some description of authors/sources.

Almost anyone "needs" one or more of the 'basic books,' (Warner/Lomax et.al.) but you'd need to know what they've got already. Most of my other recent "bargains" have been chance finds in used book/flea market excursions, and the ones I've recently bought "on purpose" get into the $50+ ranges.

I believe the Silber & Silber wordbook has been on the shelf at my local Barnes and/or Borders within the last few months, although I haven't noticed it in my last couple of trips.

John