Just to be clear: Not ALL mods have bigger batteries than you eGo. If battery life is a big deal to you, don't even bother looking at mods that take 10440, 14500, 14250, 15270, 16340 or even 18350 batteries. Some eGos have, what, 1100mAh? Out of the batteries I've mentioned, some 18350's and 14500's are close at 900mAh (even the 18350's that claim 1200mAh are truly 900mAh). If battery life is a real want/need, go 18650 mod. And do your research on the safest mods and batteries to use. There are so many to choose from. Fixed voltage, adjustable voltage, variable voltage, ones that use stacked batteries, ones that use parallel batteries, ones that use a single battery...etc...That being said...
When your eGo batteries DO have to be replaced, they'll cost you, what? $15 or more each to do so? Some mod batteries cost the same, some are less expensive (generally lower mAh or lower quality batteries, lower price). You've gotten 6 months so far from your eGo batteries, that's awesome! Many find they need to be replaced around the 3 month mark. Yes, you'll still have to buy cartomizers...unless you go all out and get yourself a Genisis type mod...where you rebuild the atomizer yourself for a VERY low cost each time you have to do it. The inital price for such a mod will not be cheap...however...myself, I spend at least $20 a month on cartos, that's $240 a year. Then I need several 1.5ohm atomizers for my bottomfeeder mod, that's at least another $50 a year. I can get an awesome Genisis type mod for $240 (plus shipping) that comes with enough wick and wire to make attys for roughly a year, then buy enough to do it again for a year for $13 plus shipping. So, inital cost...$240, plus at least two AW 18650 $26 (I have a charger already) $266 plus shipping. Still less than what I pay for years worth of cartos and atomizers. And I can rebuild the atomiser for around $13 a year from then on. So for me, worth it. For you...I'm not sure.