You can't. The pricing of paintings varies greatly depending on year, quality, condition, and artist. There's no simple algorithm to it either. You'll have to get a professional to examine them personally. I suggest you have someone examine it who is not intending to purchase them, that way there's a better chance you'll get an honest price. Someone who wants to buy your paintings will probably try to convince you they're worth less than they really are so they won't have to pay as much. If there's a local art museum, they may have someone on site to examine works of art, you may be able to get them to price your paintings.