You heard wrong. But most of us did in the beginning. A filled cartridge's longevity depends greatly on how the user vapes the ecig amongst other things. In any case 5 carts is no where near equal to a carton of analogs (tobacco cigs).
The best way to determine your cost is by how much
juice you will vape on a given day. On average I'd say that's about 2-3 ml a day. That might equal one to two packs of analogs. Yours would probably be less. Keep in mind different atomizers produce different amounts of vapor so it's not an exact science. Also different juices vape differently and as your battery charge depletes it is less efficient.
My advice is to plan on refilling your own cartridges and do some research. Figure the ml price of the
juice you are considering. Plan on using about 2 ml a day multiply this to equal a month's use. Could be less for you but that should be a good number to start with.
Next figure in the cost of a couple of each atomizers and batteries per month. Ideally you'll keep a couple of extra on hand as these things can die on you without notice. Some will last days and others will last 3 months. The thing is there is no way of knowing. You might consider a usb passthrough and a usb battery pack for the 901 to save on batteries. Bottom line, you may or may not save money. For a lot of us we feel the plausible health benefits outweigh a small increase in cost if any.
For me personally I was spending $6.50 per pack at a 1 1/2 packs a day. I use about 3ml of juice per day so I'm definitely saving money. My first three months I spent a total of $300 opposed to the analogs that would have cost at least $800. You can also check out the diy section on this forum for some great tips at cutting cost by cutting your juice. For example you could get 36mg nicotine juice in a 50ml bottle. Add another 50ml of PG or a mixture of PG with VG (for more vapor). In that scenario you end up with 100ml of juice for just little over half what you originally paid. Lots of great info here.