This site can give you the number of sketches an artist has made for a particular set. So in the sketch card section; it shows some artists with <50 creations.
Those are perhaps deemed more rarer.
So production numbers and the quality of the sketch are important.
I think that website is quite accurate, but correct me if I am wrong.
To buy, the best way to get an sketch at the right price is to ask a commission directly to the artist. With that, you are supporting to the artist and you'll get the character you want for a lower price.
But if you want to buy it at Ebay because the sketch is done in official stock, then you must see if the piece worth the price for you. But lots of sellers are charging for the sketch and the box to get their money back.
You need to check COMPLETED sales - what people ask for and what they get are different. Hall sketches have been selling at about $100 ($150 for a 2 card panel). Also, male characters get lower prices than male characters.