I would say guppys can be quite profitable to breed if you go about it right. The main thing is not to spend to much on getting a tank to put them in- a plastic container or bucket of 5gallons or more is all that is need for a trio. I would definately advise filtration, heating is also advised, but if you keep the tank at a stable room temp of 24-26 degrees through night and day then i guess you don't have to have heating- the main thing is keeping the tank at a stable tropical temp which doesn't fluctuate too much.
You can cycle the tank quickly by cloning the filter media (i can tell you how if you want, but for now i will get straight to the main points), i would definately advise cycling the tank this way as putting guppys in a cycling tank could effect the quality of the fry born and the health of the parent guppys etc.
At the bare minimum, i would advise having at least 2-3females per male, but the more the better. In a 5gal i'd only advise a trio of guppys, in a 10gal you could have up to 6, and in a 15-20gal you could have 10-13 guppys etc.
I would advise setting up a fry tank of at around 5gallons as by raising the fry separated from their parents, you will be able to ensure that more fry make it to adulthood- if you let the fry fend for themselves in the main tank, you will risk many fry (if not all) getting eaten by the parents sooner or later. Fry can also be raised in a breeding net suspended in the main tank, but only for a short period of time and only in small numbers of fry. A fry tank is much better, survival rates tend to be better in my experience when using a fry tank of decent size, and the fry tend to grow stronger, bigger and faster when raised in a proper tank set up.
Female guppys can give birth to over 30 fry a month on average, you should have some densely planted areas in the tank for both pregnant females to hide in from the males while they are giving birth, and also for the fry to hide in themselves (they will instinctively hide in heavily planted areas).
A varied diet is the key to good health in guppys, you can raise them solely off fish flakes, but this tends to not be that healthy for them (it would be like us solely living off hamburgers- posible, but not healthy), foods like krill, bloodworms, daphinia, tubifex, artemicia etc will all be readily accepted, frozen or live foods are much more nutritious and easier to digest than dried ones for fish.
You can raise your own live food for your guppys cheaply by doing things like leaving a bucket of water outside for mosquito larvae to settle in etc which can then be fed to the fish.
You can buy special foods for fry, like liquifry or powered fry foods for example (which i personally use and recommend), but you can raise the fry on crushed fish flakes too. The only problem with feeding the fry crushed fish flakes is that it can be messy and can be difficult to make sure that the crumbs are small enough for the fry to easily swallow (you don't want them choking on the food). I wouldn't advise raising the fry on brine shrimp though as it isn't very nutritious, it tends to be protein and nothing else.
The fry tank must be kept very clean as feeding fry can be a messy buisness sometimes, and stuff like uneaten food in the tank can harbor nasty infectious fish deseases like columnaris etc. I clean my fry tank twice a week, doing 2 50% water changes and substrate cleaning sessions a week (this is no big chore though as the tank is only 5gals and only takes a few minutes to do).
Whatever you feed the fry though, you should ideally feed them 3-4times a day with small portions of food.
The fry will grow at a steady pace and tend to mature at around 3months old, although sometimes a little earlier or later. All fry appear female when young, but as they mature the males will become apparent amoungst them eventually as they develop.
Guppy fry also appear very bland in colour when young, but they will develop better colours as they mature- some colours like metalic ones may take a long time to come about, even as long as 4-5months. Fry which start to develop their colour early on tend to be males.
You should separate the males as soon as they start to appear amoungst the other fry to help prevent inbreeding amoungst them (inbreeding the guppys will result in poor quality weak offspring).
When i last sold guppys i had raised to my lfs some years back, i was offered £24's for 24 guppys i had raised over a 3month period of time, who were all from the same batch from the same mother.
The key to creating desirable guppys is to choose healthy strong pure bred adults to begin with- there are many poor quality purebred guppys and mutts in the world of guppys, you've got to do your research to choose the right specimens for breeding, and take care to raise them well in good conditions etc.