there may be cheaper options, but I got a JB antennae and it works just fine. I have thought about one of the hidden antennaes which go in the faring, eliminating having to take it off and on when using the tour pak. i have heard they work well.
The J&M shortys work great great and look much better. Also tuned for the CB so no problems with SWR like the other cheap short ones on the market. They are a little pricey but worth it IMHO.
J&M -sponsor here - has shorty antenna's - I have one for CB, and one for radio - work fine!
I took the whips off my Ultra and installed one of the J & M shorty's (I don't use the CB so I left that off). So far it seems to work well. I listen to a 1000 watt college radio station often and when I leave work which is at least 30 miles from their tower I can just pick up the station which, yes I do pick it up better in my car but even that is on the edges of their signal. I did have some problem last week just driving around town and a 50,000 watt station was fading in and out but that might have been the station at the time as it hasn't repeated that behavior.