Price of 1 pot of plant plus 1 bag of soil ->  $12.50 × 2 = $25

Price of a bag of soil -> $25 – $15 = $10

Difference in price -> $15 – $10 = $5

For every set of 1 pot of plant and 1 bag of soil, he paid $5 less for the soil

To pay $45 less for bags of soil, he bought ($45 ÷ $5) = 9 sets

Amount of money spent on pots of plants -> 9 × $15 = $135