this is because when an IP is blacklisted it's generally done in a contiguous block (subnet) that are probably shared by hosts on the same virtual lan (vlan). this is to prevent mailers from jumping to another ip on the same subnet.

trunking an additional vlan into their virtual pool just for you would have constituted a major production change in probably 2 or 3 core switch layers.

the operational risk and expenditure for this kind of change easily goes into the thousands of dollars, possibly tens of thousands if they screw something up.

what exactly is "any amount of money" to you? because to them and other service providers, it means tens of thousands of dollars, not another 50 bucks.