At the market stall the cheapest goods are on the far left, the priciest on the far right.
You always buy from the left. The more you buy, the pricier the next crystal gets.
For "You pay" you add up the prices from the left. For "You get" you count how far your coins reach.
Example
For parents
What does this exercise practice?
The market stall practices adding as a means to an end and a first sense of budgeting - the child works out how far its money reaches instead of just doing sums.
Common pitfalls
It is tempting to multiply one price by the number of crystals. Because the prices rise, every single price really has to be added up.
How to practice at home
Build a little shop with counters where every further item costs one coin more, and go shopping with a fixed number of coins.