You can save 50% (or more) on the standard taxi fares, by taking a shared taxi. Even if you hire the taxi with no other passengers in it, you will still pay only 50% of a standard taxi fare (fj$100 instead of fj$200) on a trip from Nadi to Suva. Opt to share with 3 others, and your cost is only fj$25.
Here's how it works. There are shared taxi stands in Lautoka, Nadi and Suva (that I know of, there are probably more). These are manned 24/7 and once they have 4 passengers, they depart, and each passenger pays 1/4 of the fare. The taxi price from Suva to Nadi or Lautoka (or in reverse) is fj$100, split by the number of passengers.
Considering that a "standard" taxi fare is in the region of fj$200+ this is a big saving. It's a great system, and it runs well, though you might have to wait for other passengers to arrive if you want to travel in a full vehicle.
When a regular taxi goes from Lautoka to Suva, unless they have a relative to stay with, they need to return that day. They may or may not get fares on the way back. Naturally, the price assumes that they won't get a fare (though they nearly always will!).
A shared taxi driver on the other hand, doesn't need to worry about this. Once he drops his passengers, he goes to the local shared taxi stand and waits his turn... he will likely get home that night. If it's late, he might sleep at a friend or relatives house, or snooze in the cab. He always has a fare both ways.
The fare is a flat rate of fj$100 from either Lautoka or Nadi to Suva, or reverse. Shorter trips (such as Pacific Harbour or Sigatoka) will be a percentage of this.
Each taxi is full when they have 4 passengers, at fj$25 a piece. However, I generally don't want to wait that long, and if there are 2 people waiting, they may agree to splitting the fare 3 ways... fj$33 a piece.
If they are locals, you can be sure they will invariably say no, we will wait. Not because they want to, but because they think it's likely you will do what I do... say I will purchase the extra seat and pay $50 as long as I get the front seat and we leave now! It's still a very cheap trip compared to a regular taxi, so I never mind doing that.
I have used both modes of transport many times between Lautoka or Nadi to Suva, and the taxi wins hands down. The bus is cheaper at fj$18 Lautoka to Suva, but that is the only advantage.
The taxi will take at least an hour off the trip, closer to two. They will stop when and where you like for rest room breaks, or photo opportunities. And they drop you at your door, not at the central taxi stand. I would never take a bus again for such a long trip, unless I had no choice. The other advantage is that your valuables are a lot safer.
As previously noted, no travel website would recommend using these as a form of transport. However, the fact is, they do exist in very high numbers, and I doubt that they will be going away anytime soon. But use at your own risk as they are not registered! I have personally used them many times, and found it to be a cheap and fun way to get around. But if you do the same, then you need to be aware that safety standards are not high, and your insurance may not cover you in the result of an accident.