This is how you calculate it:

There are 24 spaces on the lucky board. 19 of them yield 5 points and 5 of them yield 10 points

So first you calculate the average amount of points you get with a single dice.

Then you pick an item, say Kushina, who is 2640 points. You divide this amount by the average amount of points you get per dice to know how many dice you need.

Then you simply multiply the amount by the price of a single dice.

This is the average price you get. You can inch away from the price more, if you count in 14 free dice and the occasional free rolls.