I think a cab is going to be your best option. You can take a bus, but right now the main road to the airport is still under construction so you’ll have to take the buses that run around the backside of the airport. It’s a lot longer ride and you’ll need to be sure you get on the correct bus. Plus if you have a lot of luggage that can be a bit of pain to haul around on public transportation.
That sounds about right price-wise, depending of course on where in the city you’ll be going. Be aware though that more and more taxis are installing meters which in my experience end up costing more than if you were able to negotiate a price.
Well, I answered my own question. I took the bus from the new airport ($8.00) to the old airport, about an hour. Then a cab to my destination ($6.00). Of course where your going in Quito makes a difference in the price. By the way my yellow cab had no meter that I could see.