Quartermaster, I don't know the ddo store sales of slayer count boosts, but I can probably. Guaruntee that they are crazy low.
The problem isn't the price, its the potions themselves. Slayer areas award such low XP beyond the initial run through grabbing all notes or explorers. The kills required for XP. Are huge, and by the time you get even 500 in a slayer, you have already outleved it. These potions need to double, then triple your kills when used.