If you sell 200 shares (which is half of what you bought at 2.50) @ $3 you get $600. Your base amount was $1000 half of it is $500, $600 - $500 is a $100 profit. You make a $200 profit when you sell all your shares not half of them. Anyway, this adds nothing to the meaningful conversation get_sgort_e had going on.
The point is to take some off when you have a profit. You are correct that bigger moves are required to make equivalent profits, but it's quite common for people to watch a stock run up and down the hill without action.
I usually tell myself 15-25% is where I'm thinking about taking some off...My reasoning is a good mutual fund might get this range in a year. If I can do it with a stock in a week or two, that has to be good enough. This stock is a bit different because it was so undervalued.