All prices remain at the current level, applies to both costs and income.
Configuration file: noinflation
Command line: -I
This switch turns off inflation. However, it only works from the point it was turned on into the future, and doesn't undo any previous inflation. Prices stay at their current amount, but they won't increase anymore. This affects everything, income, purchases and maintenance/running costs, and for all players too.
Use this if you frequently have too much money, because without inflation you'll get much less money.
Note that you still pay the same amount of interest, so now interest is much more important than if inflation is turned on.