Transaction fees act as a reward for the miner. Because these are the only rewards, this prevents inflation of the network since no new tokens are introduced. There are currently 5 types of transactions, each with a fixed fee. You can check the transaction examples here.
1 + 0.1 * N
NOTE: Currently the fees are static. In the future they might become configurable by node owners. This way the transaction price becomes independent from the token value.