Problem with decision making

We have a problem.

Question is Aaron and 5 of his classmates are organizing a car wash to raise money.
They must purchase soap and sponges. The group expets to wash 100 cars. They plan to use 30 sponges.

Sponges
A - pack of 5 @ $6 total
B - pack of 10 @8 total
C - pack of 15 @ $10 total

Aaaon expects that if they purchase the least expensive sponges, they will need twice the number of sponges. Write a new estimate for the cost of the least expensive sponges.

Would that be $12? -- or another answer. Tx for looking.

If they're going to use 30 sponges, I think the costs would be as follows:

A - pack of 5 @ $6 total, implies 30 sponges at $36 (6 packs needed)
B - pack of 10 @8 total, implies 30 sponges at $24 (3 pakcs needed)
C - pack of 15 @ $10 total, implies 30 sponges at $20 (2 packs needed)

So initially, it would seem that C is the best deal. But if using C sponges would require 60 sponges instead of 30, then the cost of using C-type sponges would double to $40 (since you'd need 4 packs instead of 2), making B the best deal at $24 in total.

Does that sound right?

- Steve J