It depends if you want to do short term influence or long term influence. Short term I would say even 100-200k could do the trick right now because of the low volume and low price. But if you want to affect things long term (where the big boys play) you would need somewhere around 10-20M. Take away a couple zeros if you are able to use margin websites.
How much money and Bitcoins do you need to influence the price? A Huge some of investment for one single transaction would influence the trading price market put in by investors on specific coin as posted on the market. Market investment by the consumer's into Crypto Currency Market would change the market structure dependent on the Volume of investment made. To the price as to much still it is dependent on the investors decision if he has the Million whom willing to lost for a single transaction.?