Glenn's blog on batteries, mentioned above, is worthwhile, imho.
If you are currently using something like an Ego/Vision/whatever stick type battery, you've probably got no more than 1300mah. If you are considering a replacable battery mod with regulation like a Vamo V5, Provari or similar, and you buy decent 18650 batteries (and any needed extention tubes for the mod if applicable) and charger, you'll have around twice the power available. These regulated mods allow you to adjust the amount of power (watts) or volts used to energize the coil in your tank. If you use about the same as your current stick battery, you'll get about twice the life. However, if you crank up the watts/volts on your new regulated mod, you can use up the battery more quickly.
I own both of the above mentioned devices, and they are very convenient for me to use. There are also other fine competing devices in both price categories (about $50, and about $200). The largest batteries I use (18650) have cost me between $10 and perhaps $17, depending on where I bought em and the specs for the particular battery. If you go this route, make sure you buy a decent charger that will stop when the charge is complete.