I think $5-6 is a rip off, usually they sell for $2-4 at good aquariums in Melbourne... Sometimes $1 if you buy in bulk! Most aquarium owners offer discounts if you are a regular customer and ask for one.

At least in my experience... so you can always try asking.
But you are right, it is better to breed them yourself (you don't have to quarantine them for 30 days like you do when you buy them from aquariums!

)
As far as what you need to breed guppies: A separate tank as if kept with your axolotl, they are unlikely to last for very long. The tank does not have to be big at all (20 litres?). People I know with them just seem to keep them like goldfish and don't put a lot of time into making a fancy tank. They are tolerant of higher temperatures too, and are more likely to breed at higher temperatures.
They eat fish flakes (adults and babies), but make sure you turn the flakes into powder form. Then you just sprinkle a little on top. Also brine shrimp are good and once they get to a certain size they will eat blood worms. Babies should be fed every day and adults less so. Sometimes I forget a day or two and they nibble on plants

, which seems to do them no harm...
I would say weekly water changes are fine, just like with your axolotl but of course, more when you are cycling the tank... Not sure if you need a filter or not, I probably wouldn't use one and do more frequent water changes but others may differ.
My feeder guppies are breed from a stock of 1 male to 3 females and they have babies ALL THE TIME! The rabbits of the fish world I guess. Usually get 30 at a time. I have used guppies that are a few days old to a few months so I don't think it really matters when you move them to the axolotl tank. I find smaller is better so they aren't likely to attack my axie!
If you want more detailed help I found this site, which seems pretty good or you can google around for some more help:
http://guppyplace.tripod.com/Breeding.html