I would like to find out an average Profit Margin (%) a manufacturing
company would add onto their products. In Particular, heavy Machinery
(for a commodity market) manufactured in China. A round figure is
substantial. Please also send reference of where you got this figure!

Well that depends on the order q'ty  and products.
If the q'ty can be regular like each month per order and each order
full container load, the average margin for a professional
manufacturer will be around 10%-15%.
If the product is new in maket and manufacture is the only one making
it( few cases but there is) the margin will be much higher from 30%-
50%.
I am not sure what machinery you are refering to, I am doing DIY
mahicnery for home use not industrial use. I guess the industrial one
will be bit higher profit.
I am a manufacture in China, has been in international business
business for many years.
Hope this helps