Like any business, it depends on demand, supply, does you're product fill a need in the market, and what price the market is willing to pay.
Basic economics will let you know that with a product that people want, the lower you're price is, the higher the demand for you're product will be and the more units you will end up selling as long as you get the word out and people know it's available.
You need to identify you're customers. Who will you be selling to? What will you're customers be looking for? What kinds of set-ups/color schemes will you're body match?
***An example would be:
#1 People seeking a good looking aftermarket body.
#2 People who don't have too much money to spend on an aftermarket body.
#3 People who have "enter set-up colors"
Aftermarket bodies are being sold by many mainstream companies and you're going to be competing with all the big names. However, with a good looking design, you can effectively compete by having a low price.
What will be you're competitive advantage?
You're competitive advantage should be price. Assuming you have a good product that people want, a low price advantage will get people to think " Hey that's a nice product, and it only costs _____." demand will be higher compared to a higher price, and you're sales should go up as long as you're getting the word out effectively.
How much will it cost to produce each body and ship it to the buyer?
Time & Effort
Materials Costs (body, tools, paint, etc...)
Shipping, packing materials, etc... (bodies are light, use first class mail)
That's just the basics.