Evolution is the process by which life changes over time. This process is almost immeasurable over the life spans of a handful of human generations. A million years is needed for a species to change to a new one. Other fossils are widespread. However rarity does not always play a role in the value.
Complete dinosaurs are rare.
Only a handful exist in the world. Their value in scientific terms is immeasurable. In dollars perhaps hundreds of thousands. Dinosaur bones are not rare in certain rock layers, and it is possible to purchase a chunk of bone for a couple of dollars. Rare fossils do not have to be dinosaurs.