Best coffee value

If one is a frequent coffee connoisseur, then acquiring a discount card is a sensible idea.

For the purpose of this exercise 3 establishments have been surveyed.
Muffin Break, Donut King and Michel's.

All offer a free coffee every x bought ones, but just which one has the best value?

Muffin Break give you a free one every fifth coffee,

Donut King give you a freebee at the 7th one and at
Michel's you have to wait for the eighth one to be free.

Considering that the average price is approx. $3, and converting the figures, the percentage savings are as follows:

1). Muffin Break - 20%
2). Donut King - 14%
3). Michel's - 12%.