This is a loaded question.
A bonsai is valued at what someone will spend for it.
Reasonable price for what? What are you looking for?
Cuttings/seedlings?
Starter pre-bonsai?
A tree with some age and development?
Highly refined and developed tree?
Yamadori?
What species of tree? Size? Age? Development?
All of these factor into the price.
I have purchased from Brussels Bonsai.
Some consider them expensive for some of their material, I found it reasonable for what I purchased.
Other vendors:
Wigerts Bonsai
Eastern Leaf Bonsai
Cedar Rose Nursery
Before investing a lot of money into a tree or trees, get some starter material and learn to keep things alive first.
Everyone kills trees as they are learning. So starting with lower cost plants is wise