Ebay is either very poorly priced or very, very well priced. Typically I've found that popular species for bonsai (trident maple, juniper, pine, azalea, etc.) are very highly priced, while less commonly popularized species like pomegranate, wisteria, bald cypress can be more reasonable. I've often seen great deals on 'local pickup only' listings, as you're competing with a much more limited audience.
I think the commonly given advice to not spend too much money on your first couple trees is good. Since you have kept a couple alive for some time it sounds like, I'd say you're ready for some material that excites you. I don't know about you, but I am very bored of mallsai, and I didn't really feel like reading stacks of books about bonsai until I had a few trees that I really loved looking at.
Having nice material makes bonsai worthwhile, whether that's a finished tree or a future specimen.
I would just setup a penny jar and every time you have a bit of money or change, stick it in there. Join a local bonsai club and find out what grows
really well in your area. For me, that's azalea and japanese maples. In Florida it might be Bougainvillea and Ficus. Where you are, well, I don't know. Often, bonsai clubs will have auctions selling off material whose owners have gotten bored of the trees. You can find very good deals at such auctions, and, as important, you'll make friends with the original owner and can always hit them up for guidance as to its care and perhaps the direction that they see the tree going in. How much the tree will be varies, but do consider how long someone has been working on a tree, their time, care and effort, not to mention costs of soil, water, fertilizer, etc.
My rule of thumb is generally if it's nice material, with some character and vigor, it'll be around $150-300. But you can start finding some really nice trees for that price.
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