If you're trying to buy it all at exactly the top or the bottom, you'll always be disappointed.  Buy your stake in chunks.  Decide what is a good price for you, then put in buy orders in advance, 1/10 or 1/20 at a time, or whatever division works for you, and assume it will go lower than you think.  You'll spread your risk, and sometimes you'll get quite a bargain.  Sell in chunks as well.