D-Link DIR-880L + Mac => monthly data usage stats possible?

I'm pretty frustrated with the "crappy software with no update ever" business model of D-Link so I'm thinking about maybe buying a Synology router instead (already have a NAS from them and I like their OS) but I'd rather spend $0 with some Terminal Voodoo or something like that

Ahhh yes... You've seen the light! Consumer grade routers are just too limited for what they do. You need a proper router that has the ability to measure your bandwidth.

As for Terminal "voodoo," no. There's no magic command that allows you to simply measure the amount of data that has gone out to the internet. 75% of it is how you architect the hardware setup. If not your router, you need to somehow put your computer (it's network interface) between your router and your network so it can "count" all of the network packets going in and out. It's like having a bouncer at a bar with one of those click counters but instead of putting him at the door, he sits at the bar - there's no way he can accurately count the patrons coming and going; you need him at the door.

but I'd rather spend $0

Then you want pfSense. I'm a huge fan of Synology and make heavy use of the NAS gear. As for networking, I swear by pfSense for my routing. First off, it's free. Their model is either to purchase their appliance or purchase managed support. You can install pfSense on a cheap Core 2 Duo. Your biggest expense will be a quad Ethernet port if you want to have a DMZ or handle multiple LANs. I put one because I put my guest WiFi network on it's own network and kept them separate from everything else.

Here's just one of the sample Stats reports (Traffic Graph) that comes out of the box.

However, based on your premise for starting this project, no more unlimited bandwidth, you can set up traffic limiters to prevent going over.