I called Boss awhile back about this. They'll charge you $10 plus some change for a new black thumbscrew, which includes the cost of shipping.
Yeah... it's the shipping that kills. I usually order some jacks and knobs and stuff to offset the shipping a bit when I need something specific from Roland.
The problem with that is... if you have some spares laying around you might be tempted to use them to fix more broken Boss pedals, then you'll need more parts, then more pedals.... and that's really the start of the downhill spiral