I have pretty good luck buying cheap upgrades to Biz Class when I'm flying on Delta marketed, KLM/Air France operated flights. I used to think this was something you could just do when checking in for a flight in Amsterdam and requesting a miles + $ upgrade. However, recently I have been able to select a business class seat when checking in for San Francisco to Amsterdam and Tahiti to LAX flights. The Flying Blue site offered me upgrades in the range of $700-$900 one-way. Combined with a c. $1000 round-trip Y ticket, this seemed like a great deal for business class.

I was able to do something similar when connecting to a Virgin Atlantic flight from London to JFK. (I opted for Premium economy for about $250 that time.)

Connecting out of Amsterdam, I used to get offers in the range of $400 and 30K miles. I'm having a hard time burning through my stash of miles, so this seemed like a great deal. It's been a few years since I have been able to take advantage of it and I hear that it no longer works for Delta operated flights, like it used to.