I had total success using Sonar AS. To me the cost is relative to effectiveness and Sonar or similar products seem to be the best for watermeal by far. It's certainly a lot of money for such a small quantity of liquid, but it's very concentrated and long acting. Also you need to factor in the value of your time because there is very little labor involved in applying Sonar.

I mention my experience with watermeal in this thread:

http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=89101#Post89101