I don't claim to be a Vegas Guru, but here's how I think about it for an office pool with a big $5 a week on the line.
If a team is playing at home after a Bye, I will give them the advantage on a low spread. So, if team X is coming off a Bye, is playing at home against a team to whom they would normally be an underdog and is a less than six point dog on the spread, I'd pick them to beat the spread.
So, to answer your question, I think of a home field after a bye being worth about three points.
The history of a team coming off a bye is also important. The Pats, for example, tend to do very well after the bye.
On the road, it's not that clear to me. And it's game by game. Last week, for example, I took Atlanta over the niners even tho they were on the road and dogs and did pretty well by it.