It has to be commodity grade 9mm. 115 grn fmj standard velocity, brass case, can be had for prices ranging between $10 and $15 at discount locations, both B+M stores and on line. Now, what used to be the cheapest handgun ammo, 158 grain .38 special, is running $16 and up.
Right now, at Graf's, an online dealer, you can get $10 steel case wolf in 9mm, or $12 federal 9mm. The lowest price of .38 is $16, the same as a .40 SW.
Graf's has a $5 flat rate shipping charge.
This post is assuming two things, first is that you're talking about centerfire, factory loaded ammunition, second is that you have internet purchasing available. Local stores commonly will have $10-$11 9mm commodity grade ammo once in a while, too.