The price of mSOL is calculated as follows:
Price of mSOL = total_staked / tokens_minted
As the protocol cannot mint new mSOL without SOL being exchanged for them, only the total staked amount is going up (for any new mSOL minted, the same amount of SOL, at the current price of mSOL, has to be staked and also joins the total staked amount).
This means that the price of mSOL is going up each epoch relative to SOL as long as staking rewards are distributed for the SOL staked in the protocol. If you keep mSOL for a year, its value against SOL will have gained 7-8% (Marinade's APY at the time of writing).