The inflation % is, AFAIK, based purely on difficulty level and game turn. It gets higher as you play on, and there's nothing you can do about it. The total amount of inflation is determined by multiplying this percentage by all your expenses (unit cost/supply, city maintenance, civic upkeep.) For example, if you had a 10% inflation rate and you were paying 10gpt unit upkeep, 20 gpt city maintenance, and 20 gpt civic upkeep, inflation would be 0.1 * 50 = 5gpt. If you lower those costs by building courthouses or something, the total amount you're losing to inflation would go down.