That kinda assumes that spammers are paying for their own hosting.
There are two ways of doing it:
1. Use a compromised host to serve the spam target pages
2. Rip off credit cards to buy hosts to serve the spam target pages
Given the lifespan of URLs in spam (which need to constantly change to dodge the blacklists) then either method would mean 100% profits for the spammer without having to change their existing methods - they don't spend *anything* on hosting.
This is just another money grab from legitimate users - I don't think it's (financially) worth me renewing any of my .com/.net domains any more...way to piss on your own chips, guys...
(Option 2 does have the 'plus' side that the idiots who are actually buying from spam links get their credit cards ripped to the max - hopefully they'll learn not to buy stuff from fscking spam!)