I was thinking it would be cool to plaster the whole outside of my coffee cup with RGB LED's (WS2812B).

Then I measured the cup and did the math.

Diameter = 3.24"
Height = 3.83"
Circumference = PI * Diameter = 10.178"
Surface Area = Height * Circumference = 38.98 sq. in.

Flex PCB Cost = $15/(sq. in.) * Surface Area
Board cost = $584.70

 That is quite expensive and I don't even have the components yet!

However, that is three copies of the same board so if i did a 1/3cup design I could combine them to cover the whole coffee cup.

Area = 12.99 sq. in.
Cost = $194.90

That's still expensive for a design I have not proven yet.

How about trying this.

Area = 2 sq. in.
Cost = $30
However I receive 3 boards.
Therefor Total Area = 6 sq. in.
Coverage = Area of PCB/ Surface Area of Cup * 100 = 15.39% 

Now I just need to figure out the layout and the routing.

Enjoy a photo of the coffee cup I used for my measurements.