In future miners will be rewarded almost exclusively with transaction fees.
The idea is that bitcoin is widely used with millions of transactions on each block, so those fees will be enough.
So theoretically, if Bitcoin really become adopted as we expect, the prices are very low now as the reward is 12.5 BTC for each block plus fees (every 10minutes). The next halving will reduce to 6.25, which will probably lead to a great price increase
I don't know what exactly will happen when the mining reward gets so small that the vast majority of money is made from tx fees, but I can tell you for certain there will not be millions of transactions per block. Currently with segwit I think it can realistically handle around 4000 tx/block. Hopefully they will scale on-chain in the years to come but it's unlikely to ever be more than tens of thousands of transactions per block. Though if on-chain fees are like a dollar or something per transaction (in today's dollars) that should be plenty to incentivize miners to secure the chain.