Engagement Photo Prices

So my fiancé and I are going to be getting our engagement photos taken within the next couple weeks. I have a friend who's doing them for us. I'm driving us all to a couple locations and taking care of any outfits, props etc. She's just taking photos, edits, and giving us access to them, probably by letting me just get them from her computer with my flash drive. I told her I'd pay her for her time and for doing all the photography stuff. She doesn't do it professionally, just as a hobby, so she didn't give me like a set price to pay her. In total we'll probably go out and to about three different times and locations. And knowing her, I'll be looking at probably 100 pictures or so that she'll edit and give me. So I was just wondering what seems like a decent price to pay her? I was thinking like $40, but I don't know if that's good, too little, etc? I'll be buying any prints I want so that's not an issue. Just paying her for services. Any advice would be appreciated.