$0, only book if R>0 and "buy up to" W. (which is basically a co-pay). The only time I go above W is if thats the lowest available fare. If you're asking how much I've spent on the "buy up to" W versus the lowest available fare, that's a tough on since I don't really track it. I usually take a look at its around $400 round trip perhaps.
I don't travel for intl for work much anymore, mostly for vacation so I can be flexible.