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:
EM Wave EM-M43002 Tri-Band 150-162/450-490/763-870 MHz Mobile Antenna - 17.7 in - NMO - Spring Base
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:
PCTEL Maxrad RBGC30 Magnetic Mount - NMO - 3 1/4 inch - Rubber Boot Base - 30 ft RG-58/U Coax Cable - PL-259 Connector Installed
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.