I've heard that leaving supporter doesn't effect your custom domain, so you could, in theory, get supporter for one month, set up your custom domain (which would probably be around 10-20usd) and then leave supporter before it pulls for the next month. I don't know if anyone has tested this though!
As a side note, I've been dying to have a custom domain for a while now, but supporter is 60usd/yr... and I don't have that kind of money
I've heard that leaving supporter doesn't effect your custom domain, so you could, in theory, get supporter for one month, set up your custom domain (which would probably be around 10-20usd) and then leave supporter before it pulls for the next month. I don't know if anyone has tested this though!