I see. If you can't find someone to agree to work that way (Scrum), and don't want to pay by the hour (which has its positives and negatives), you might also be able to negotiate based on each overall asset.
Even asset prices can differ, though. While a tree sprite might cost $10 or less, a fully animated 2D main character could reach $30-$50+. Factor in a bit more than that if we're talking 3D, especially if the animations that are needed are complex.