What you are actually paying for
A 3D printing price is rarely just plastic. Material is usually a smaller share than machine time, and on top of that come file setup, finishing and packing.
Typical cost items
Material (grams used incl. support), machine time, setup (file review and orientation), finishing (support removal, sanding), risk of failed prints, plus packaging and shipping. The final price is provided after technical review.
Why two similar files can cost differently
Orientation, layer height, infill and support requirements change both time and material significantly. That is why an accurate quote needs a technical review rather than a guess from model volume.
Getting a realistic quote
Upload your file, state material, quantity and intended use. You get a quote with the assumptions written out, so you can compare like for like.