The price of a jar of pigment is 19.50 plus tax though, and the pigment set would have had tax too, so it seems reasonable to me to take the price including the tax and divide that by the number of vials.
I note the original poster is from England, where the pigment set cost more than it did in the US (because our prices are higher AND they already include 17.5% sales tax), so I definitely think making an allowance for sales tax is fair.