Have to agree with jmaul if you are actually running through the life cycle of the small batteries that fast you'll never be able to afford eGo batteries if you have to replace them that regularly. A battery MOD provides access to much cheaper batteries that last much longer and you only have to replace the battery not the switch/housing/etc.
That said, I'd give serious consideration to a pass thru of somekind. So you are not wearing out batteries prematurely using the same ones two or three times a day(still even three cycles a day should last three months might need a different manufacturer or vendor). Even if you only use PT at home that is one or more cycles you save your battery from having to go through making them last over a longer time frame. Plus they make great backups when a battery fails prematurely on you. If you have manual batteries make sure they are sealed, some brands arent on occasion. Most premature battery failures are juice getting in that hole manual or auto, autos have to have hole.
Cant really comment on atty lifespan as I switched to cartos almost immediately, mainly because I felt like I was constantly filling the carts mainly out of paranoia over trashing the expensive atty. It's possible the more expensive atty's you saw were the so called 'mega' atty's. They go with a 'mega' cartridge that is supposed to hold more juice, but that is the only difference I'm aware of. Regular carts are not long enough to work with the megas if you end up with some of both.