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How are you suggesting they manipulate prices? While there are a number of security flaws, none of them allow counterfeiting. You can't increase the supply beyond what it should be no matter what you do. You can destroy bitcoins by sending them to invalid accounts and cause deflation that way, although I have no idea why you'd want to. You could get over 50% of mining capacity and frequently use it for the 51% attack, causing people to lose trust in bitcoins and making the price fall, but again, it's not going to help you.
Not prices, but the cost of production. With increased prices normally you would have increased mining efforts which, through competition, would make cost of production rise towards the price. If you control the amount of mining, you can avoid that and keep the mining effort at the same level. Your profit (price - cost) then stays high.