I dont breed and sell Guppys but if the shop is selling them at $5, they possibly paid a max of $2.50
Now with food, power, time and a heck of a lot of other costs (Including even the petrol to take them to the shop) there is not too much money being made by the breeder and I would suggest that they are doing it at a loss
To make say an average income of say $20,000 as a part time income and allowing for say $100 per month for power and $20 for food, the breeder would have to breed and sell over 8500 fish.
$5 is really cheap in my mind (BTW I have never sold a fish)