Some of the pizza parlours and other junk food outlets charge approximately AED 5 per delivery. Certain restaurants have a minimum order amount and their delivery is free.
Like IndoMLA has pointed out, 5-10dhs as a tip is quite standard even if there are delivery charges because the way I see it, the delivery charges go back to the restaurant but the tip stays with the guy who rides the bike with loads of food on these crazy roads and manages to bring our food on time (most of the time!)!