As a person that DOES sell guppies to pet shops - I can assure you the breeder does NOT make any money.
Firstly a guppy that I sell for $1.50 to the LFS (and they will not pay more at any costs!) will be sold on for at least $5.00 - this is to get ALL the costs covered (by the pet shop): Rent, salaries, kiwisaver, insurance, phone, electricity, advertising, interest on loans to get the shop up and running, any deaths (as fish not always take kindly to go from one tank to another with completely different water parameters) etc etc ......
The $1.50 will of course NOT cover MY cost of raising said guppy - I am selling him cause I cannot just "compost" the poor fellows. This is what most LFS are happy about.
If they buy imported guppies then they often have the so-called "guppy disease" which kills the fish very quickly - or at least it will kill all the guppies that I sell to the LFS - as mine wouldn't have any resistance to that disease.
So if any fish shop has bought "lovely imported guppies" (as in females with coloured tails) - they have no other choice than to sell them for 10 or 15 dollars each - so long as they can keep them alive.
I would strongly recommend that if you want guppies for keeps that you buy of a local breeder that has raised several generations in NZ - WITHOUT buying any off-shore fish in!!