system
A chat on subreddit /r/pcmasterrace
human
Isn't this much more expensive than a raspberry when you consider electricity?
gpt
Tl;Dr: if just looking at electricity costs, yes. If looking from a performance stand point, no. 

Long answer: It depends more on your internet speed if you have less than 10Mb coming in, then yeah a raspberry pi would be great. Not sure if the newer ones support 1gbps, but if so then you could theoretically have up to 500mbps coming and going and still be good.

 If you're going to make it a firewall, you'll want multiple network interface cards (NICs) to separate traffic. Depending on the nic you may be able to use VLANs, but then you're cutting the NICs speed by however many VLANs you have (1 divided by # of VLANs). Also, a lot of people say, oh well just buy a USB nic, and yes you can, but you have to get a very high quality one (read: comparatively much more expensive to non quality products) because besides being limited by USB speeds, the manufacturers just lie and most products I've used simply won't get above a few Mbps. I'm sure in theory the NICs can do it, but in practice they're junk. 

Then you have actual processor speed to account for, a laptop will almost always have more power than a Pi, which is useful if you want a fully functioning firewall with DNS/ad blocker, maybe a VPN connection as well, etc. 

For a final thought, the laptop would be far more resilient compared to the Pi. I've bricked a lot of RPis by pulling the power without a proper shutdown. It's very easy, especially when using SD cards which aren't actually built for the purpose of serving an operating system. A laptop's hard drive could still fault if you cut power, but that's only a real concern if power is lost when the device is powering on or off. In this case, provided the battery is still good, it'll act as a backup if the power goes out, something an RPi doesn't have.