Zep: Sonar (Avast!, Fluridone, Whitecap, etc) is a very cost-effective treatment under the right circumstances, AND if "selectivity" or "localized impact" are NOT high on the user's agenda. Fluridone is a relatively broad-spectrum chemistry, but is generally suitable only for ponds and lakes that have very little if any flow-through (transient water).

Regardless of which herbicide-options you're comparing, never assess their "cost" by comparing their pricing per pound or gallon. To do so would be equivalent to comparing the cost of a .22 to a 30-06 simply because they're both firearms. The true expense of any herbicide treatment should be compared to other treatment-options based on the cost per month of "control" that each option offers.

Example: A treatments with Product A might cost $100 per acre, but yield only 2 months of control; while a treatment with Product B may cost $200 per acre, but it provides 8 months of control. So, which is more cost-effective treatment? In this example, Product A is twice the cost of Product B over an 8-month season, even though it is half the cost-per-acre per treatment.

That said, fluridone is one of the most cost-effective treatments available for many nuisance aquatic weed species - with the following caveats. 1) The treated pond isn't used for irrigation. 2) The pond doesn't experience much or any water-transition for 60+ days (<my opinion). 3) The user's objective entails removing most or possibly all of the pond's aquatic vegetation (<depending on the plant species that are present and their relative susceptibility to fluridone).

Lastly, be aware that multiple formulations of fluridone exist; liquids, pellets and granules. These formulations (especially the liquids) may vary dramatically in their concentrations of active-ingredient (a.i).

Example: 1 gallon of a 4-lb a.i. formulation contains the same amount of active-ingredient as is found in 8 gallons of a 1/2 lb a.i. formulation. There are additional formulation-factors to consider besides simply the product's % or wt of active-ingredient, but that's a long story. Similarly, fluridone pellets and granules are generally comprised of 5% a.i. formulations, but they also have unique formulation characteristics and a.i. release-rates that might make one formulation more suitable than another for a particular circumstance.

I hope this somewhat answers your question, rather than further confusing you. Needless to say, it's a bit more complicated than simply looking at a product's price-tag.