I could relate to your cost concerns, I'm having the same issues with my Njoy.
One word of caution: Using any other brand with your Njoy WILL void your warranty, which is one of the things that make buying an Njoy worth it in the first place.
That being said, unless you return several atomizers in a short period of time, it SEEMS that they won't really try to check.
The most popular and economical way to refill is to use e-liquid, by topping of the cart, or by taking the almost empty filling, let it "soak up" the e-liquid, and placing it back into the cartridge.
Some of the more popular suppliers IMO on this forum are Johnson Creek Smoke Juice, Jane's Juice (which is primarily sold on Ebay), and Janty's Juice.
Now, it's very important to stress that currently their are some problems being reports with Johnson Creek Smoke Juice, which the company reports they are working on fixing, and I would not recommend purchasing that IMHO right off the bat as they are working on resolving the problem. I just received some and it killed my atomizer within the hour, but some people have reported no problems while using that juice.
Also, poke around the forums some more, and use the search, since this topic is constantly being talked about.