Indeed there is a big price difference, an experienced character artist who makes custom work knows what their work is worth. There is much more than just making the character - knowledge takes time. Anatomy/style/color/technical constraints, then there is client feedback, rework, admin/emails etc. Much more work involved = higher price + experience + cost of tools + business costs etc.
To answer your second question, if you buy a stock model then you can customize it as you want if you have the skills obviously. Regarding copyright the model should be tagged as Royalty Free license type. If you have purchased a model that is Royalty Free and is actually an Editorial (Iron Man for example), then you can request your money back from CGTrader. Lots of artists create base meshes which you can use or there is plenty around online for free.