It did not take too long for me to figure out the following:

The total cost of tea ($) / the total weight (g) = the cost per cup ($)

Let's say I get a $ 15 pu erh cake (add $ 10 for shipping and
handling, so the total cost is $ 25), we use 7g from the cake for each
brew, and it should yield good 7 cups of tea (7 infusions). Use our
formula as follows:

$25 (the total cost of the cake) / 357g (the total weight of the cake)
= $ 0.07 cost per cup

Yes, it only costs 7 cents for a nice cup of Pu-erh tea.

Now let's try an expensive $ 40.50 pu erh cake. (the same $ 10
shipping and handling, but the cake weights 500g). A quick calculation
reveals the following:

$50.60 / 500g = $ 0.10

Wow, only 10 cents?! I believe an average tea bag would cost me about
5 cents for each cup of tea - almost undrinkable though.

Now it makes sense to buy better tea at a higher price. First of all,
it tastes better. You want to enjoy the tea as much as possible, your
time is golden. Cheap stuff does come cheap, but do you really have fun
to drink it?

Above formula works for any Pu-erh teas, raw or cooked.