The quality of spokeshaves is mainly determined not by what they are made from, but how they are made.
Better quality (and more expensive) spokeshaves are not simply cast, painted and then sold, they are machined to improve fit, performance and usability.
It is important, for the precision tasks required of a spokeshave, that the parts fit together correctly. The blades of some spokeshaves are inserted upside-down, with the bevel up. Whilst spokeshaves can be used like this, bevel down is generally preferred.
You will also find that the lesser quality spokeshaves are not smooth and the pieces rub together, meaning they cannot work efficiently. Another problem you may find is that the blade is not sharp enough.
These are issues which can be fixed by replacing pieces for better quality ones, working and sharpening the parts or buying a better-quality model in the first place.
To spot a better-quality model look at the finish/ paint on the tool and how well the pieces sit together. It should be quite simple to see if the tool has been machined or not. The better quality tools will be more expensive due to the resources and time used on their manufacture.