I can find quarter horses all day long for $100-1,500 each. It will depend on age, bloodline, papers, training.. Etc... The price of horses will go down due to winter coming up. People really don't want to feed horses during the winter, so this is the best time to buy.
On average, a quarter horse will cost you about $1,000 to $6,500. However, a quarter horse can go for as much as $20,000 or more. You can go to http://www.equinenow.com/quarterhorse.htm and look at the different prices.
There is no real average because it depends on registered? Bloodlines, age, level of training, suitability for different riding levels, also mare, gelding or stallion. also, depends on the market in your area. The average backyard horse that may or may not be registered, is suitable for advanced beginners, is approximately 9 and is gelded will run $500-$1500, possibly more if it's child safe. A registered gelding with good bloodlines , same age, and a minor show record will go $2000-$4000 and same scenario for a horse that shows successfully will go from $4500 up, depending in what kind of shows it was successful in.