Building a mod is a great way to save cost....plus you get to decide exactly what you want it to be like.
If you don't have the patience (like me!), invest in a Reo. They're extremely well built and the fact that they are bottom-fed devices means that you don't have to ever worry about dripping or cartridges again.
The larger Reos use the 18650 batt, while the mini uses the 14500.
The only downside with them is that demand greatly outweighs supply at the moment

When Rob (the creator) makes a batch, they tend to sell out extremely quickly.....in a matter of minutes in some cases.
I've been using mine for the past 6 months and it's the only thing I use. IMO, bottom feeding an atty produces the best results in terms of draw, flavor and vapor production. You get all the advantages of dripping, without the hassle.