I'm in the 7% group, spent about $2500 on a pine two decades ago, sadly it is no longer with me. Have plans to spend up to $1300 on a Satsuki azalea this spring. I have set aside the money. but have not picked the tree yet. If you are in the Midwest, Rick Garcia is now in Indiana, and has a nice stock of satsuki available.
To expand a little on points
@Sansokuu brought up. Her categories should include #6 beyond #5, where the public collections reside. Your professional artists and public collections would fall into either category 5 or 6, and these collections are very important for helping to spread the hobby. The Chicago Botanic Garden has a large bonsai collection. I
n the category of supporting the idea that bonsai is ART, the Lynden Sculpture Garden hosts the Milwaukee Bonsai Foundation collection of bonsai trees. Both Chicago and Milwaukee are respectable public collections.
@IzzyG has been a generous donor to the Milwaukee Bonsai Society, & MBF and the collection at the Lynden Sculpture Garden.
As a general rule, yamadori and nursery stock will need anywhere from 5 years at a minimum to 25 years or more to be brought to "exhibit ready". In fact very little stock is truly "exhibit ready" with less than 10 years of refinement. Prices of bonsai reflect a number of things, one component is number of years in cultivation.
It goes without saying cheap trees, say less than $500, on the average, will not be "exhibition quality" in terms of being ready for shows of the caliber of the National Exhibit, or even the yearly show at the Chicago Botanic Garden. However, the same less than $500 tree might be just fine for local club shows.
There are exceptions, and some small shohin and smaller trees can be obtained less expensively and developed quickly. But on the average, good trees take TIME, and TIME adds cost to acquiring a tree.
One can start with $20 nursery stock or "free" yamadori or "urban yard-adori", put a decade in, and have a true exhibit worthy tree. Which can work out. Though in my experience, 95% of my personal $20 projects ended up on the burn pile as total failures. Every once in a while a worth while "winner" comes out of the effort. I have a number of $20 projects going right now, some a decade or more old. None I'd call successful enough to post.
And because so many of my less expensive projects are mediocre, I have discovered that starting with more expensive, better material, gives me more joy maintaining the tree and putting more effort into the rest of the collection if I got one or two that are "better" than the rest. Besides, I'm not getting any younger,
Spending more lets me skip the "nurseryman phase" of bringing a tree up to size to begin "bonsai". Raising seedlings is not bonsai, it is nursery techniques. You need mature stock to "do bonsai" on.
So instead of buying a bunch of cheap projects, save up, buy one or two "better" projects that inspire. Save myself a decade or more of grunt work.
Maybe I should make some cute washi tape or swedish dish cloths. All affordable And below double digits. 
