If they were things you couldn't easily get on your own, and you plan on ever seeing these guys again, then I would be very generous. The National Concierge Association (and of course I realize you are not going to be in the US) recommends:

but if the concierge gets you theater tickets and a table at a top restaurant after the show, $20 to $30 might not be inappropriate.


I would have no problem giving this or more if they really have provided me with something I couldn't or didn't want to do on my own.