What is the aproximate break-even point for buying the mug refills in comparison to buying soft drinks in disposable cups?

So to start this question you find the prices of the two types of cups and create a basic formula for them. For the mug it would have a base price of $2.95 and a refill price of $0.50. For the disposable cup it would cost $0.85 a piece. 
So for the mug you would take the base price of 2.95 and add it by x amount of refills to find the break even point of the two. So the formula would look like 2.95+0.50x. The other would be 0.85x. Now its just a matter of guess and check to see how many refills it would take for the mug to be cheaper. i hope this helped!