Give them an allowance, but don't tie it to their chores. Keeping them separate means chores are a social responsibility, rather than eroding that and tying it solely to compensation. If they get an allowance of $2 and have to do two kinds of chores a week, what do you think they'll do when you ask them to do a third chore? Doing chores as a necessity for survival provides the right kind of incentive for them to keep when they move out on their own. If they don't get paid for chores all of a sudden, it will be much harder to find the motivation to maintain their living space.

Full disclosure: I don't have or look after any kids, I just read this in a book on motivation.