Good question! I've divided my answer into these two points

1. Effect of lower prices on Bitcoin miners

If the price per Bitcoin drops, there will be a point when mining gets not profitable anymore. This break-even is difficult and depends on the individual costs of each miner to mine one Bitcoin. The first miners getting problems are those having high electricity costs and high salaries to pay for their employees. The cheapest conditions are actually mainly in China and Russia, where energy and salaries are low, but also renewable miners, which have special contracts to get a surplus of enery from hydropower stations (like in canada).

The result of miners leaving the network means a decrease of hashing power (if in addition more miner leave the network as new miners coming in) and the decreasing of hashing power wil result in a lower difficulty (the difficulty is a result of all miners confirming transactions and the more miners join the higher the difficulty gets (the difficulty is updated every 14 days to adjust the difficulty for the avarage blocktime of 10 min). Also the block rewards (12,5 BTC per block) are divided among more participants, if more miners join). If the difficulty decreases the block rewards will be divided among less participants and this means every miner gets more BTC.

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2. Effect of lower BTC prices on transactions

The transaction fees will decrease most likely as we have seen this in the past. Remember in January 2018 it was other way round: increasing transaction time and costs due to a very high BTC/USD rate. But there was another issue: the unconfirmed transactios. So the miners confirmed the transactions first who had a high transaction fee and the others had to wait. Now we have only around 2.000 unconfirmed transactions, in January 2018 we had around 200.000 unconfirmed transactions.

Lower transaction fees also mean that the miners (who are getting the transaction fees besides the block rewards) earn less BTC as if the transaction fees are higher.