adkimery
Veteran
Ok, you guys got me with some of your examples. I didn't originally consider staying loyal to a 1 day gig and passing up a month's worth of work to do so, or a wedding vs. career-booster movie gig.
The idea of finding a suitable replacement is cool, but just for the sake of argument I thought of a couple issues. What if your unique vision/working method is what someone wanted, and any replacement, no matter how good, is simply not going to be YOU? Also, again with our original example of a $500/day guy working for $200, well now you have to find ANOTHER guy of that skill level/caliber (who, likewise, probably commands a similar rate) who will work for the same rate in your stead, or else the replacement would be a less experienced person, which is not what the client originally signed on for.
If it's that importnat to have that guy then find a way to pay him his typical rate (assuming money is the only reason he's moving to the other gig) and/or reschedule your shoot for the next time he's available and hope it doesn't fall through again. Just because you don't get the guy you want doesn't mean great things can't happen. I mean, Molly Ringwald reported turned down the lead role in Pretty Woman.