The shaft one wants for breaking is stiff and strong
The shaft one wants for jumping is light with significant flex.
I can't see how anyone would want a J/B with the physics of the shafts being so different.
I use a $60 Lucasi jump cue and an $80 break cue (that happened to come with my pool table.)
The cues are probably less that drinks at the local bar for a month, all combined.