I breed gouldians, and I love them. I always marvel at how beautiful they are.
your typical green back will run you about $50-70, in Canada, so your price sounds quite good.
I prefer them over the other colours, such as yellow backs or blue back as they tend to be more hardy.
males have the more bright prominent colours, where as the females are more diluted, but non the less still very beautiful
females will have a black ring around their beak, providing their face is not black, if it is then you will have to look at her colouring on her chest to see if it's more dilute or vibrant.
males tales tend to be a bit longer than the females.
they love eating mealworms, finch seed, and some canary seed.
good quality egg food can be added for extra protein, but only at certain times of the year
they can eat vegetables, and some fruits, but love leafy greens. they tend to be more picky when it comes to eating healthy food, but if you continue they will eventually grow and love eating it!
Gouldians typically moult early spring(Canada & U.S.) and moult hard. they look and feel terrible. they often sleep most of the day for about a month or so.
nearing the end of their moult, you can start adding egg food for protein.
mating season is usually between July-December
I have a male blueback which cost me $150.00 cdn. ( I just couldn't resist)
IMG_2702_zpsf153bc62.jpg Photo by bmckellar735 | Photobucket
they are highly susceptible to air sac mites, just so you are aware.
It's not uncommon to have any type of finch with air sac mites, due to them being transferred when being fed as chicks. they aren't a problem until they are really stressed or not healthy. If you hear clicking, or wheezing I would seek out an avian vet in a hurry.
you can house them with many various species of finches, and canaries.