I actually paid for two PDA apps: thinkDB and iSilo.
thinkDB, as with any good database program, is incredibly versatile, and I found myself using it for so many things, I wanted to upgrade... so I paid up. iSilo I bought just because it was cheap, and provided a pretty good .doc reader interface with a free html-to-pdb converter utility, so I could make my own docs.
But I think for a PDA app to be commercially viable, it either has to be a can't-live-without-it-once-you've-got-it sort of app (like thinkDB has become for me), it has to be $15 or less (like iSilo), it needs to be targeted towards a high-budget professional market (like doctors, lawyers, etc), or it has to be an add-on or tie-in with a popular/useful desktop app. (and of course, a fairly generous -- 30 days or so -- trial period, for people to get really hooked on the app is a good idea)
If you've invested the time and money in FileMaker Pro on your desktop, having FIleMaker Mobile (or whatever they call it) makes more sense than a less expensive mobile DB app like thinkDB. If you have wireless internet, having a Palm app that will play nicely with your desktop email program would be good...
There are just so many good free programs available for the Palm market (and probably a lot for pocket PC also), that it's really hard to think of any need that isn't already covered by more than one free or shareware software package.