I am asking about day to day, and in within runs 

A lot.  Sometimes, I'm around 9:30, and sometimes I'm around 11:30 both on the same run & with the same perceived effort. 

That's a more complex question than it first appears.  On really hot days, my easy runs may be 45 seconds slower per mile than on cool days.  Windy days can have a fairly negative impact as well.  Did I race a few days ago or are my legs relatively fresh?  Also, it takes me about three miles to get zoned into my pace.   Assuming that weather is not an issue and that my legs are relatively fresh and that I'm past the three mile mark into my run, my pace won't vary much.  I'll be within about a 5 second window of 7:40.  

Again though, all those other factors can come into play.  My first run after my half marathon was an 8:34 pace.  Looking at my log from July shows a lot of stuff in the 8:00-8:15 range.

For me it can vary by about 50 seconds depending on how tired or rested I am (all other factors being equal).  If I'm rested, 8:50 pace feels easy with little effort.  If I am tired, 9:40 pace is the same amount of work.

I ran hard but short both days these weekend (6.8 miles and 4.5 miles).  I anticipate tonight's easy run to come in around 9:10 pace. 

I actually wore a watch on my run yesterday.  I mostly just timed from beginning to end so I could average out a pace (9:21), but I did clock myself from mile 4 to 5.  I was just under 9 minutes.  I can tell you that my first 2 miles are always going to be slower.  I'm sure they were around 10:00 ish for that run. 

I always start out slower until I really get warmed up, which is after about 2 miles.  My pace probably picks up by 45 secs to a minute.  I would imagine this is fairly typical with runners. 

My easy pace can be all over the map, granted even if it feels easy and is getting too fast, I put it in the category of a GA run for me. LRs, now those ones I make sure I keep in a specific range regardless of how easy or good things feel.