I have one of those 48cc motor kits on a cruiser. It cost me $177 to my door, and $60 for the bike, and took about 4 hours to install. It is a fun toy for short distances (actually a 7 mile commute may be perfect for it), but I much prefer my regular commuting bike to get to work and back. I also looked into electric bikes, but since they start around $1,000, I figured I'd go with a gas motor first (total cost around $300) and see how I liked it. So far so good, but I still am not completely sold on its reliability (only 50 miles on it so far). Check them out at
www.motorizedbikes.com if this would work for you...