it is on any phone (not just the iPhone) on either AT&T or Verizon (I don't know how other providers handle activation fees) and it varies between $0, $18 or $36.
Dial 611 from your current phone and ask your cellphone provider, they will more likely know.
I paid $18 for my upgrade on my personal (single) line back in July, but my mom paid $0 on her line because she is the main account holder out of 4 lines, but I have heard that it can be as high as $36, but I don't know what determines those fees.