Update: May 26 patch notes indicate yet another chapter in the ongoing Broker pricing adventure. Pricing still maxes out at 50 million plat per item, but you can spend up to 100 million plat per purchase. Note that the Large Plat Values feature is no longer necessary since all possible sales prices can now be set with the standard pricing keypad.
From the May 24 patch notes:
Whether or not this is a permanent rule change, it's accompanied by a total suspension of all brokers and plenty of lively discussion in general chat on every server that I've visited today.
Anticipating that there will be at least a short period of time with brokers reactivated along with a 50-million max pricing limit, the Large Plat Values feature has been retooled to match this new policy. You can download the new Set Prices window immediately and begin pricing your wares, even while the brokers are out to lunch.

In the official forums some players were reporting some sort of glitch when subtracting plat from a price, although I wasn't able to duplicate the bug. You should be able to increase and decrease large plat values without any issues. If you find this not to be the case, please let me know of course.
It should also be noted that all possible prices can now be set, for the first time, using the standard keypad. So the Large Plat Values widget is not necessary at this time. But if you're anything like me, it'll take a while to get out of the habit of going straight for that page when setting up prices for anything higher than a few plat.