Well, I'm not sure, but it seems as if you get to choose the probability of each item being landed on in the prize wheel. Simply assign a probability and off you go, all the while making sure that the total probability doesn't exceed 1.
If your goal is to break even, then you're going to have to create the spin wheel such that the Gem Cost Expected Value (the mean gems lost) will be equal to the cost per spin. Anything less and you will lose money. Anything more and you will earn money.
There's no shortage of ways to answer this question. Try this method, for example:
You have 6 Items that are going to be on your prize wheel. The area of your prize wheel must contain all 6 items. Let's construct a prize wheel such that the Gem Cost per unit Arc Length is equal (probabilistically) at all points along the circle:
The total arc length of a circle is 2*pi*r.
The Gem cost per arc length is (Gem Cost) / (r*theta). This value needs to be the same for all arc lengths such that the sum of all thetas is equal to 2*pi radians.
I don't know if I lost you in all of my words, but yeah... Good luck.