"Cost" encompasses things like effort, loss, and sacrifice too.
For probably much of cybercrime you don't pay for these servers. Many are usually security compromised servers that were put up for legit business and left some config file out in the open or some other silly mistake. In the case of phishing, they are hijacked and by the time anyone notices, enough emails have been sent to create a lot of damage for the good guys, and a decent profit for the bad guys.
As always, something will only be done to fix the unbearable lightness of committing a cybercrime when there is a realization that the price being paid for those that have the power make a difference outweighs their benefits.
Usually, the emails are from public leaks. So, free email list.
The hosting? More often than not, we find phishing panels on hacked web servers exploited through vulnerable Wordpress plugins or whatnot. So the servers are free.
Sending the emails? Usually a shitty PHP bulk mailer uploaded to the same compromised server as the phishing kit.
The effort on a lot of this is incredibly low, the cost is “time”, etc. public mail lists, infra compromised using public exploits, etc.