Budget art supplies for a project.
Art supplies divide into things you use up and things you keep, and mixing them distorts the cost of a single project. Surfaces and paint are consumed; brushes and knives serve many projects, so they are amortised across the number of projects they will last. That gives two useful figures: the cost properly attributable to this project, and the cash you actually need at the shop counter, which includes the full tool purchase. The per-piece figure is the one to use if you intend to price or sell the work.
Amortised tools
Tool cost charged = tool spend / projects the tools will last for
Per piece
Cost per piece = (surfaces + paint + amortised tools) / pieces made
No, and doing so is why first projects look prohibitively expensive. A well-kept set of brushes serves years of work, so charge it across that span and record the full outlay separately as cash flow.
Cost per piece is the floor, not the price. Add your time at a rate you would accept and a margin for the pieces that do not sell; materials are typically a small fraction of a sustainable price.