Its done at a coating facility. You supply them with the aluminum. And as far as costly goes depends. I did a $80,000+ order with them to keep the cost reasonable so that I can make enough profit.
You can look up coating facilities and see what they would charge you, but first you need to know what your raw aluminum will cost you. Its always cheaper to order in large quantities, otherwise you better just stick with unisub and the like.
For the coating itself...its a polyester based paint. But as I found out...you cant just use any polyester paint. It took several months for them to get the formula down and there were many issues such as the color being dull, the coating being sticky/tacky etc. You might want to search for a coating facility that has done coatings for sublimation.