The upgrade should not take much time.
I would 2-3 days should be enough to upgrade and test everything on local machine and then deploy it on production server.
The time consuming tasks would be the ones which invove ladder climbing from 1.5 to intermediate versions. As far as i know, there is no way to directly upgrade 1.5 to 2.7 (the latest one). You have to upgrade to some intermediate versions and
then gradually reach the latest one.
Cost depends on the person you engage to do this for you. It varies a lot from person's geographical location too.