Can you now see why not much money is needed to maintain a certain price?
For merely 10-20M $ (an irrelevant amount for miners) you can easily prop up the price by 500-1000 $
on a single exchange, which obviously affects the rest of the market as well due to arbitrage opportunities.
Keeping the price above a certain level is even less expensive and the miners don´t even need to
inject new capital, because they can simply refrain from selling block rewards.
See last 30 days trade volume:
https://data.bitcoinity.org/markets/volume/30d?c=e&t=bMore than 200k BTC are traded per day. That's more than $1.7 billion per day. Throwing away $20 million to pump the price won't change it much. And I doubt the miners would or could do this often. At least many miners would need to combine their money to buy Bitcoins, but then it would be more profitable for the miners who don't collaborate, so it won't happen.