Pretty much the only boards that seem to be valuable in the $100+ sort of way are IBMs (SSK, Industrial, Model F, etc), Apple Extended Keyboards (especially the Salmon versions), and the Northgate Omnikey.
There’s no way you’ll get $100+ for an Apple Extended Keyboard unless it’s in mint condition, with orange Alps switches (orange switches are more desirable than salmon), and even then it’s a slight stretch; they’re just too common. And there are plenty of other keyboards that can fetch high prices.
As you say, Omnikeys get expensive. Other very well constructed Alps boards like old-logo Dell AT101s, SGI AT101s, Wang 724s, and similar can also get spendy in good condition, as can rare Alps boards like Xerox Docutouch keyboards. Most blue and green Alps boards go for steep, especially in good condition, for instance from Zenith, Texas Instruments, Tektronix, or Leading Edge.
Several types of IBM boards get pricey: Model F ATs, F122s, “unsavers”, and the extremely rare 4704 boards; SSKs, industrial Ms, M15s; beam spring keyboards; various Japanese boards.
Rare Old Topre boards (and new Topre boards for that matter) can be pretty expensive. There are more of those in Japan than the US though.
Certain old Cherry boards fetch very high prices, especially ones with dyesub keycaps. The G80-5000 split ergonomic keyboard is particularly sought after.
In general many types of ergonomic keybaords like Maltron, Kinesis Advantage, FingerWorks Touchstream, DataHand, etc. can go for quite a bit.
Many types of hall effect keyboards sell for quite steep, and they don’t seem to show up on ebay as much as they used to.
Many types of rare boards from the 60s–70s can get bid up very high on ebay. Just not too many people around here are interested / pay attention, because they’re usually not easy to convert to use with modern computers. Check HaaTa’s flickr accounts if you want to see some awesome crazy things.
If the goal is to simply flip keyboards, thrift shops would be equally profitable for just about any model. I've never seen more than $10 worth of boards in a single shop, and a decent vintage mechanical should go for $20-$30 in most cases.
If the goal is to make money, flipping keyboards is unlikely to be especially profitable. It takes way more work than just doing some other job. If the goal is to try a bunch of keyboards, and then sell whatever you don’t like to bankroll the hobby, then thrift stores, flea markets, and e-recyclers are a great source, and I wouldn’t worry too much about finding “rare” boards. Just buy it if you find it interesting, since the price will usually be peanuts, and then resell whatever you decide you don’t want to keep. I wouldn’t expect to make a great profit, but many keyboards can be sold for at least $20 + shipping, so if you get them for $1–10, it’s at least not a money-loser.
Definitely don’t try to make a profit flipping boards bought on ebay or similar: it just takes way too much time for any profit, probably yields at best like half minimum wage.
If typing experience is the goal, I say nothing compares to a Model M,
It’s all down to personal preference.
I do think a list of a "Top Ten Vintage Keyboards" would be nice, especially for newcomers. It would be useful to have a good, general, central reference
Top 10 is impossible. But anyway, that central reference exists:
http://deskthority.net/wiki/
