I quite like this idea... I like the idea of server-backed dynamic economies in general. I'm fairly certain we could hook into the Jobs and Towny plugins to facilitate something like this automatically but I'll leave it up to Clue to determine whether he wants to bring it on to Towny specifically since he's more involved with the community than I am.
~ @LordInateur
I love this idea, but if this is done, it would be best if players could depend on it not changing frequently. Keeping it dynamic makes it really easy for staff, especially techs in the future, but if it changes daily, it may be hard for town staff to understand how effective their money is, especially if a sudden bout of inflation occurs. I think the upkeep should then be based on a monthly or bi-weekly system (since taxes can only be changed after 14 day warnings).
I also think the upkeep costs for the next fortnight should be be available for 14 days, enough time for towns to change their upkeep prices if necessary. That means that the server would create the next upkeep for the period after the next.
Example: fortnight_1_data creates upkeep_3, which is seen throughout fortnight_2 and applied through fortnight_3. It's a bit more work, but it would make town staff happier if any sudden strong bouts of inflation occur because they could raise taxes accordingly.