When is the best time to travel to FIJi, suggestions where to stay? what is average budget for everything?

Try: Expedia.com and lonelyplanet.com ; fodors.com
Depending on where you live you can safely bet on several thousand dollars unless you're sleeping under a palm tree or the beach. Travel to Fiji is good just about year round. Go to the library and check out travel guides/books on Fiji that will help you - also go to a travel company and check their rates.