not to put too rude a point on it, but every plastics shop sell those barrels for that or less. As does bunnings, and other major hardware stores.
assuming your the seller, if your getting your price point based on what your local hbs may be selling them for, then you'll never them for $15. Unless you have someone else on your street/suburb that doesn't have access to any stores as mentioned above.
In fact i can get 25l and 30l barrels and cubes from a local "bit of everything" store for $11. ($10.95 actually)
Good luck, but i reckon you'd have to be selling them for $5 each for someone to be interested considering the available competition and other outlets that sell these.
sorry if this is harsh.....just woke up and haven't had my coffee yet.
EDIT: people often make the mistake of thinking these barrels are specific beer brewing vessels and therefore must have an element of exclusivity about them. they are just water vessels that are probably sold more to campers looking to take water away than to homebrewers..