I would say $9.99 but $15 is good too. If I find something that genuinely helps me out, spending under $10 allows me to buy it without feeling guilty about spending money without asking the wife. Not that I need her permission, but in our relationship we clear most non-household purchases with each other if they cost more than $20. If I'm spending $10, that's like spending my lunch money. :)

I would also trial 19.99 to see if I could get more revenue. Experiment! Start high, run a sale a few weeks in and see if it makes a difference.