The differences between nursery stock and GOOD bonsai stock can be subtle but can also be worth the difference in price. Nebari is one factor. Short internodes in the trunk, taper, and movement are a couple more things I also look for. Nursery stock is usually grown as fast as possible in order to maximize profit. That usually means long internodes and no taper. Much depends on the quality of the bonsai you intend to grow from the trees but often nursery stock will need to be trunk chopped really low to start making trunks with taper and to get viable branching in the right spots.
Converting nursery stock sounds simple to anyone who has not done so. The reality is many years of cut and grow, layering, etc and a great outcome is never assured, even for experienced growers.
Good stock is worth extra because of the years it will save you redeveloping less desirable trunks and branching. Recognizing the differences is another thing altogether.....
Just because a plant is sold at a bonsai nursery for a higher price does not mean it will have all the attributes you want. Growing good stock does take longer and more effort than growing for landscape trees but some 'bonsai' nurseries use the excuse to over inflate prices of sub-standard stock just because the market will pay. Unfortunately inexperienced growers are caught up in the middle of all that.
The pots these mass produced bonsai are sold in are relatively cheap but still add a few $ of value to the package so always expect to pay a bit more with a pot.
There is probably also a premium built into the price for the skill and experience of potting the tree into a bonsai pot. Many newbies are happy to pay for that as they are generally terrified to do so by themselves. You are obviously past that stage so probably did not need to pay that premium. Does the nursery have a stock section? That's where I'd be looking. Prices should be relatively cheaper without a bonsai pot or the potting up premium but generally the stock plants at a bonsai nursery should be selected or developed with bonsai in mind. Again, you will need to sort the wheat from the chaff but should offer better choices than most general nurseries where straight, vertical trunks are valued over taper and shape.
I'll wait till spring and take some more substantial layers from local trees,
Sounds simple and many newer growers enthusiastically promote layers as a great way to get good stock. My experience does not support this. Landscape trees tend to have rather straight branching with little taper and usually have quite long internodes. Even the best Japanese maple branches I have found turned out to be rather disappointing once they were in a pot and the thicker the starting material the harder it is to convert to desirable.
Maybe I just have higher standards now but I am yet to see a great bonsai developed quickly from a landscape layer. Some great bonsai are layered but usually from existing bonsai which have been developed for years to have superior taper, bends and shorter internodes.
Its an unspecific green Japanese maple.
I have found that 'unspecified green JM' can make better bonsai than most of the more famous named cultivars. Also every seedling is a new and unique variety. Some are as good or better than the existing named ones.