You could check if they fit 510 or RN4081 cartomizers. Trying to think of the most cost-efficient way that doesn't involve filling (since very few 510 cartomizers are prefilled). You could buy a pack of prefilled 510 cartomizers from LiquidXpress and if those don't fit then try a 4081 to 510 adapter (99 cents) from Goodprophets). And note - if they DO fit that does not mean you have a 510 or RN4081 - you could have a model that would fry a real 510 or Rn4081 in its charger and vice versa, but cartos only care about threading.
Re refilling (and it's not really "refilling" because you don't run them dry or you will singe cartomizers - you keep them damp). But for the sake of calculation, assume a full refill... Juice costs about 25 cent to 1 dollar per ml. Like a 3ml sample or a 5ml bottle may cost $1 per ml but once you find a juice you like and buy bigger bottles it's more likely to be 50 cents per ml and if you use huge bottles of the cheapest juice or make your own juice you could get it down to 25 cents per ml.
So while search for your favorite juices it may cost you $1 to refill a cartomizer, but once you find some favorites and buy bigger bottles (like a 30ml bottle) you should be able to get that down to 50 cents per refill of a 1ml cartomizer.
I use thin clear juices, 80PG/20VG or 90PG/10VG and I avoid juices that are dark or cloudy and drop flavoring sediment (shorten the life of a cartomizer). I can run about 20ml through a single cartomizer before it is degraded to the point where I want a new one. For me that is 7-10 days using one cartomizer (actually for me that is 14-21 days because I vape a pair, alternating between them)
I do recommend manual batteries - harder to damage or kill with juice leaks and easier to get a good drag/hit every time. And of course it helps to have quality cartos with soft cap, designed for refilling and well-structured inside to survive extended use.
A different model to look at...
I recommend you look at SmokelessImage Volt which can also use cartomizers from its twins Bloogplanet and Halocigs G6 and from a different style at V4L.com (where most of my manuals are from). These wil not be compatible with what you have - just suggesting an alternative. And I recommend 78mm manual batteries which will make 120mm ecig with 320mah of charge (what you have now probably has 180mah of charge). The SmokelessImage Volt and Bloogplanet cartomizers are very nice and I think Halocigs G6 may use the same cartos. SmokelessImage juices are 80PG/20VG and I found my 2 favorite juices at V4L (WOWbacco and WOW Vanilla) and those are thin and clear and work great in cartomizers. I shop the sales and get the juice at a good discount and even its normal price is under 50 cents per ml (43 cents per ml) .
I keep the carto damp and can get 7+ days use out of one (14+ days for me, using it a bit more than 1/2 the time)
