When I was starting out I observed what other local artists were charging for their similar sized works. With watercolors the size of the painting matters more than how long it took you to paint it, or even how good you think it is.
I show and sell my paintings in 3 different parts of the country; Indiana, Cape Cod and southern Florida. I can get higher prices for them in Cape Cod and FL than I can in IN, so the region matters too.
Finally, it seems that if the paintings fly off the racks faster than you can replace them, maybe they are priced too low. However, if they are priced TOO low, people think they aren't worth much.
In other words, pricing your work is real pain!