If they are day curtains, I doubt you will be tying them up, but there isn't any other way to prevent it from flapping around, even the heaviest curtains, unless they are very stiff - it's like a sail, however big and heavy, it traps the air and moves.
Not sure how much tassels at joo chiat costs, but if you get them from wholesalers, it should not cost more than $5 for the cheapest ones or $15 for the better ones.