I have been mailing for over 16 years.
Let me give you a little advice.
Do not pay anyone for IPs without fist testing them and making sure they are not blocked / blacklisted by the domains that you want to send to.
Just because your IPs are not blocked by the big domains, doesn't mean you will inbox all of them. Being able to inbox requires more than just being able to rotate IPs.
Most domains are going to rate limit how much mail you can send per IP. So if you want to send high volume to specific domains, you will need a lot of IPs.
Unless you are starting out with high quality data, you are going to get your IPs blocked / blacklisted, when just depends on the quality of the data. So whoever you get IPs from, they must be able to replace them. You want to have a very specific replacement policy in place, so that there are no questions as to when IPs can be replaced. The ideal situation is that you can replace IPs monthly, however that is no always possible. Never accept a deal that is longer than a 2 month commit and if you mail GI, don't accept anything other than monthly.
I don't ever pay more than $100-$200 per month for a server, depending on it's specs. If you get IPs in volume, you can often get a server(s) thrown in for free. I don't pay more than $1-$1.30 per IP / per month / depending on volume, for clean IPs.