While growing up at home (I'm 23 for perspective, so during the '90s), my mom set out a $1 tip. I always felt kinda bad. In college, we would call out fairly frequently (a few times a month) for pizza or Chinese. Since there would be several of us chipping in, we would usually each put in a $1 tip, so tipping around $3-4 on a $15-20 meal. Now on my own and on a more stringent budget, I've only called out once in the last year and a half, and even though they got my order wrong (price-wise), I still tipped the guy $3 on a $17 (should have been $12, but it wasn't the delivery guy's fault) order.
Especially if they're older (as in not a teenager), then I almost feel bad for the delivery guy/gal that that's the only work they could find, and so try to tip relatively well. I figure that even if I'm on a budget, their's is probably tighter.