A good multiband scanner antenna and a magnetic mount is going to run you $60 - $70.
There are absolutely some cheap Chinese antennas that will do what you want, but I won't recommend that stuff.
$40.95 for a good antenna. I'm running one of these on my work truck:
Tri-Band 150-162/450-490/763-870 MHz IP67 Rated Roof Mount Antenna
theantennafarm.com
Getting 15 feet of cable with a BNC connector to match your CB/Scanner is going to take some work. There are some cheap Chinese mounts that come with what you need, but my own experience is that those brands use cheap connectors that are poorly installed. I hate recommending things like that to people.
This will do what you want, but will cost more:
Magnetic Mount Black with Rubber Boot Base, 30ft RG58/U Coax Cable, For use with NMO style antennas
theantennafarm.com
You could probably save some money by going with a different kind of mount, but that's a complex choice.
It's gets expensive, but the antenna is the most important part of your setup. Spending money on a good antenna/mount will pay off in the long run. It's up to you to decide how much it's worth to you.