@spaingaroo the least reasonable amount is $100. However, depending on your monthly profit expectation, you can invest more than that. I have come accross people on steemit with account capital worth $10,000. for example, this guy started with a little below $10000, and you can notice that in the second chart in this article he posted. https://steemit.com/crypto-news/@williams858/another-all-time-high-in-dash-mining-difficulty-be-cautious-in-buying-dash-mining-contracts-with-gm
...and as he indicated bitcoin mining contracts are the most profitable. Micro hash pays more profit, but they recently started and investors are trooping there by the thousands daily.