Puzzle

In this puzzle, submitted by Jean Marie Fade, we invite you to find some unusual and interesting ways to number 20 things in sequence.

The Schlumberger center in Angola has 20 offices that each need to have a number placed on the door.

The office manager will purchase metal numerals and nail them to the doors. The cost of the numerals varies depending on the digit:

So, for example, putting the number 6 on a door would cost 6 Kwanzas. The number 13 would cost 4 Kwanzas. The number 10 would cost 11 Kwanzas.

The only requirement is that the numbers be in consecutive order. What is the least expensive way to do the job?