You are correct. The problem is called an initial condition problem. You are given an initial criteria and then use that to predict the cost of any number of purchases.
You set it up properly, solving for x initially given 8 units. Now you want to know the cost for 2 units or 100 units or any number of units, your new unknown variable (x) is the number of units.
for example
Cost = Ax + 60
where x is the number of units and A is some constant. You first need to determine A, which is what you did. The answer is then
Cost = 30x + 60
make sense?
