Unfortunately good and cheap don't really go together in Perth. When are you planning on staying? At the moment, accommodation in Perth is very hard to find.
As a Perth-ite myself, I can't think of any suitable places off the top of my head... but if you're just visiting for a holiday and not a specific event in Perth, I'd highly recommend getting out of the city and heading south (short stay) or north if you're staying longer. almost 6 years ago
I agree with Brenton, Perth accommodation is not cheap, and very rare!
We found the cottages at the Old Fremantle prison fantastic. They were affordable, available and close to the places we wanted to go to.
We wanted to be close to family and friends as well as Rottnest Island, Penguin Island and the cafes of Fremantle.
Another great website that I use for accommodation (everywhere) is www.agoda.com almost 6 years ago
Hopefully this reply isn't too late, Lisa and Brenton are correct, Perth real estate is tight and expensive. I agree with Brenton in that it depends what you are here for, if there is something specific in Perth that you are here for, I hope you find somewhere (probably need to be a bit out of the city), otherwise go North or go South. There isnt another capital town like Perth but they do have a lot to offer.
South - Busselton, Margaret River, Denmark, Albany, Dunsborough etc. It is a long stretch of coast, beautiful blue water, white sand little towns and Margaret River is WA's wine region with some very nice wineries.
North - Coral coast of WA is again small towns with amazing scenery and a lot to do. The pinnacles (2 hrs from Perth) Geraldton, Kalbarri (fishing, canoe down murchison), Monkey Mia (dolphins), Exmouth (snorkelling, diving, swim with whale sharks, fishing), The kimberleys and karijini (Mitchells plateau, kings cascade etc). The scenery is more Australian out back and very picturesque as opposed to Perth being a city.
Hope that provided a bit of clarification. almost 6 years ago