I just recently returned from a trip to Fiji and I can tell you have a lot of options when it comes to where to stay. The cheapest places to stay are close to Nadi Airport. Although they are cheaper than the resorts, they are basically an airport hotel, and you will not have access to a beach or lagoon. You best bet would be to stay in the 3-3 1/2 star range for hotels. We stayed at Crusoes Retreat and it was very nice. It was not a 5 star resort,but the staff and the guests were all very friendly. Its pretty far away from the airport, it took 2 hours by transfers. It is actually the first resort ever in Fiji. But if you like peace and quiet, then this is the place for you. If you are looking for something a little more updated then I suggest something the the Fijian or the Warwick. They are more expensive, but they have more resturants and bars and shops.
Fiji is beautiful, and I highly recommend that you make it one of your destinations.