When the popularity of the American Girl dolls arose, the problem of how to keep them clothed (reasonably) also arose.
The company came out with their own doll dress patterns and these have become quite collectible. If you can find an uncut pattern, they run between $25.00-$80.00. I found one at a book sale one time.
In addition, the major pattern makers (Simplicity, McCall’s, etc) also came up with their own series of 18″ doll clothes/furniture/ accessories patterns. These can run between $12.00-$50.00.
Of course with both, condition and rarity will raise the going price.