Inflation, as in the "real" world, causes prices to go up as time passes. There is a patch for turning this feature off.

• In versions prior to 0.7.0, the patch is controlled from Configure Patches > Economy.
• Versions 0.7.0 and later have this feature under Advanced Settings > Economy.
• You can also edit the Openttd.cfg file and use the following syntax to disable inflation:

[economy]
inflation = false