Price is determined by the marketplace. There are no rules against giving art away or charging what the market will bear. So, buyer beware. For the most part, the market works like this:
Given the equal size and quality of two paintings, the artist with the biggest reputation/record of sales will command the higher price.
Prices increase as a result of successful sales, public exposure, and especially, museum purchases.
For artists of note, especially deceased artists, fewer pieces available means higher prices.
Bigger is more expensive.
Given the same artist, paintings cost more than prints and drawings.
There is no correlation between price and the age of work in good condition; sometimes earlier work is more valuable, sometimes not.
Prices tend to rise over time.
The art market is subject to inflation; when the money supply is large and demand is strong, prices go up.
The most boring thing people say about price is “Back in 19__, I could have bought a painting by _____________for only $___!!” (Fill in the year, name of now-famous artist and unbelievable low price)
– Mike Dominguez