Anyone know how to increase this overall? It's almost always pathetically low.

There are 7 bribe entries in Game Settings (Gameplay/Settings). You may want to try an alter these settings. The http://www.creationkit.com/Category:Settings doesn't shed any information on what they do or whether these are used in Skyrim.

Type bribe in the filter box

Oh, wow. Thanks, I'll play around with these!

Not sure which is the most balanced way of approaching it, but fBribeScale certainly makes a difference. Somewhere between 5 and 10 seems to provide a reasonable challenge.