Paint shops are in business to make $$$.
Their equipment costs $.
Their shops cost $.
Their employees cost $
There are a ton of regulatory fees that they have to pay in order to remain open.

And that's just to open the doors.

Now, comes the material.

Paint ain't cheap, and if it is, I don't want it on my car/parts.
Paint is like many itemsyou buy, you spend more for smaller quantities, hence your spoiler will cost more $/oz of paint than my hood/car/etc.

An alternative is to find a paint supply outfit that will mix the paint for you & put it in a rattle bomb. A can costs maybe $25 dollars.
If you're painting plastic, they can even add the right goo that keeps the paint flexible.

There's a place in Sacramento that will do just that & I have had really good success in matching/painting various parts.