I've only bought Biz/First class tickets personally twice..
once on our honeymoon CPT-JNB on BA where biz class was $20 more than econ.. (though I could have gotten this on an award anyway, we had redeemed First class AUS-CPT/JNB-AUS)
The other time was an AONE6 RTW from Egypt. That was expensive, but much cheaper than point-to-point fares would have been. It also earned me Emerald status on QF for two years.
All my other FC/BC trips have been awards, or one biz trip to India.