I agree, the tree has potential. It's especially nice that you cannot see the graft. Awkward grafting makes a lot of this cultivar unsuitable for bonsai. You have to be patient with these trees; they grow slowly. Chop back, except for what you want to try and air layer, then see what grows. Next spring put it in a big grow flat. The tree, along with your styling techniques, will show what it wants to become. To achieve ramification, allow the spring shoots to grow out, then remove the leaves that are not growing in the direction where you would like to have secondary branches. Usually those growing up or down. This will encourage the growth of the secondary branches. But, leave a downward growing shoot that is placed at the end of a branch, because when it extends, it will probably do so in a way that will add movement to the branch. I think the asking price is in line with what is usually charged for this cultivar.