*Too much detail in the story needed to be eliminated, i.e. your handsomeness

But I think you used that side story to distract us, right?
Working backwards, let's determine the cost of the items you are buying before 10% was taken off.
x * (1.00-.10) = 13.50
x=15
So 15 is the number that will be helpful from here.
buy one and it costs $5, ten costs $10, and one hundred costs $15
however, if you replace the words with digits, the problem can be simplified as
1=$5
10=$10
100=$15
And if you look long enough, voila...
Are you buying numbers?