The DMG lists a mercenary's pay rate as 2 sp per day (p 149). Sounds reasonable, right? I thought so, until I looked at p 108 of the PHB. For 2 sp a day, our poor mercenary can afford a single poor-quality meal, and he has to spend every other night sleeping in the street. The mercenary leader is a 2nd-level warrior who costs 6 sp, +3 sp/level above 2nd. That means that a 1st-level fighter who sets aside 10 gp of his starting cash could hire a 3rd-level warrior and 5 1st-level warriors to accompany him for three days. That seemed a bit cheap for me. Furthermore, I wanted the pay rates to fit in with the amount of gold characters have floating around. Table 5-1 in the DMG outlines how much gold a character should have at a given level. If you check it out, you can see that around 10th level characters can start rather easily raising armies to deal with their problems.
Of course, the DMG tables assume that you have to outfit a mercenary with equipment, but that seemed a little silly to me. But then again, at 2 sp per day it would take the average mercenary about 9 months to afford studded leather, a large wooden shield, and a longsword if he did nothing but save up his money and starve. Again, that didn't jive with how I've done things in my campaigns.