So... Zero
I remember once a Con staffer giving me a death glare because I discounted the charity posters we were hucking at a con to one staff member who'd spent time chatting with us, asking what charity we were supporting (the Con's chosen charity for Malaria) but only had his last £5. But refused to discount further the already discounted posters my artist friend was selling as well. The guy murmured some very rude words and stormed off, shoulder barging me in the process.
Con Staffers really do think they're special, rather than volunteers giving up some time. I mean, it's nice if a VA says thanks or whatever, but you aint a paying customer, bucko.