I have had jailbroken iPhones since generation 1 and have not noticed any significant battery draining. I would check again to see what you're running that's sucking up up battery.
Since you always JB the iphone and dont use it w/o it, you dont know the diference.
Actually yes, JB 4.2+ (not sure about 4.1) drain the battery, how much, like 15-20% more,
Today I did the JB for 4.3.1, since 4.2 im not using JB on my iphone coz the battery drain.
Daily I do the samething, Drive to the Gym (~15 mins), at the gym while im in the bike watch a series episode (~40 mins), do some workout listening music(~30 mins), drive back to home (~20 mins), by the time im back @ home my battery is @80-85%
Today, back to home, using my iphone 4 4.3.1 w/ JB, the battery was @ 68%.
If you dont care about that draining, its ok, but sadly it matters for me.