How a price is reached
Three things drive the number: machine time, material weight, and whether the part needs work beyond printing.
Machine time is the largest factor and it is not proportional to size. A tall thin part with fine layers can take longer than a chunky one twice its volume. Material is charged by the weight of the finished part plus its supports. Finishing is quoted separately because the effort varies enormously with geometry.
Every quote states the modelled weight and the estimated print time, so you can see how the figure was arrived at rather than being asked to trust it.
Why volume changes the per unit price
Setup, calibration and the tolerance coupon are one off costs. Across one part they are the whole price. Across two hundred they nearly disappear. That is why a batch quote is genuinely a different per unit price rather than the single part price multiplied.
When we will tell you to go elsewhere
Above roughly a thousand identical simple parts, injection moulding usually beats printing on unit cost even once tooling is paid for. If your quantity has crossed that line we will say so.