There are many ways in which to answer this question. Let me make a few points.
First, not all works of art are worth a lot of money. Some will never be worth a great amount, even if it is a good piece of work. There are other factors that have a bearing on price.
Second, when it comes to price, there are several factors: skill of artist, public willingness to purchase, and level of rareness.
The skill of the artist does play a role. Some artists are singular in terms of their skills and creativity. People will pay a premium for artists like Vincent van Gogh. He was groundbreaking.
The public willingness to pay for art also hugely impacts price. In a strong economy, people will pay much more and in a down economy, art work will be discounted.
Finally, the most significant factor in the price of art is how rare a piece of work is. Originals will fetch the greatest price, because there is only one. Signed lithographs also are very costly, because there is a limited amount. The less of something, the more the price.
You've heard of many artworks from famous artists such as Claude Monet, Edvard Munch, Vincent van Gogh, and etc. going for millions to even billions of dollars and people are thinking that they can imitate the same artworks as theirs, so what's so special with them anyways? You must take into account that these are artists with different art styles that caused an uproar during their time frame, negatively or positively, and these are artists that you can never get back again. So this makes their artworks very rare, a collectible, and comes with quite a large price tag.