I sure hope your $30 computer can handle 1gb of symmetrical bandwidth.
The computer isn't $30, that's just the cost of the components you need to use the computer as a router. As for whether it can handle it, my router (a WRT54GL) has a 200 MHz single-core CPUs. Anything it can do, my PC can do with 2% CPU utilization.
Anyway, my point is that most people who desire to load a custom firmware onto their router probably already have several computers anyway, and at least one that is always on for one reason or another. I personally just don't ever turn my computers off. I also use MythTV and so, even if I did regularly turn them off, I'd keep one running 24/7 for MythTV. A lot of other people have always-on computers that function as file or print servers. As long as this is the case, why buy a $150 router and fudge around with custom firmware when, with only $30 of parts, you can use that computer to do your routing, NAT, firewall, bandwidth management, and anything else Linux can do? Why spend hundreds on a device which isn't even going to do what you want until you void the warranty?
As for prices, here's what I found on microcenter.com:
gigabit switch: 5 port for $20, 8 port for $25
gigabit wired PCI card: $10
wireless b/g/n PCI card: $15
So that's $50 for gigabit. If you don't want gigabit it's even less. Plus you've got $100 left over. If you want to spend another $50 you can get a 16-port switch. Maybe blow your last $50 on a more expensive wireless card with a special antenna you can put wherever you want. For the same money you'll end up with something far better than a simple router.
Obviously there are cheaper routers, but the routers people are buying to load custom firmware onto aren't the cheapest ones. Not only do they need routers that are Linux-compatible, but they also require larger flash memories, and they want features like USB ports so they can use them as file and print servers, and attach external storage so that they can keep log files. At some point you have to realize it makes more sense to just use a spare computer for the task.
...and let's not forget that every time you flash a new firmware onto your router you risk bricking the device. Then you'll really wish you'd spent your money on something more useful.