From: Mosquito Bite, Texas
Whenever you buy units, your UCI reflects the total cost. As the total cost you've spent goes up, there is a % increase based on some increment of money. That increment is different for each country and can be found on the "Economy" screen. France, in 1805, has a factor of 130. So, if you've spent 260 on units, you will see a 2% increase to the cost of the next unit(s) you buy.
The only way to lower the % is to not buy anything, or buy less that the amount that the total decreases each year. But, it takes a VERY long time to lower it in some scenarios, so sometimes it's just best to get what you need quickly, then let your spring levies fill the gaps for you. I usually don't buy anything after the first couple of years, myself. I capture artillery and build cavalry and specialty units, then just let the levies provide me with infantry. (Playing France here.) It generally works.