Almost all my air travel is leisure and I usually upgrade to FC with miles (and now status). However I have occasionally purchased discounted FC seats when the price difference was not too much.
For example, I flew ATL-JAX about a month ago, and the difference between FC and Coach was about $50 each way. Given the expedited boarding, additional legroom, and other FF perks I would earn, I decided it was worth the extra expense for the FC seats. I have also bought FC tickets for inter-island travel in Hawaii under similar circumstances (short flight, FC was only about $50 more).
Granted I can't usually afford to pay out of pocket for FC on long haul flights, but it certainly is in the realm of possibility to buy FC tickets without spending "4-5 times more than economy".