When you run a single miner, you contribute a very small portion of the total energy, or the total hash rate, to the network. You are one of many. Accordingly, the probability of finding a block is very low. If you do find a block, the reward is very valuable, but until that happens a lot of electrical energy and maintenance has to be invested.
To make the mining business more predictable, there are so-called mining pools. Miners can join one of these pools via the internet. If a miner then finds a block, the reward is divided between all the miners participating in the pool in proportion to their delivered hash rate.
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