If you put your daemon in VPS, you only pay $5 a month
Exactly!
Masternode investors can set up and operate their masternodes using VPS, which is more profitable than running on their own computers.
This and also the fact that you can't run multiple masternodes on the same computer, like 10+.
Yes you can, you just need an additional interface.
They are $2 per month on Vultr, with a max of (3) interfaces per VM.