Go for the extra DNS servers. $10/mo is a small price to pay for a fault-tolerant setup you have full control over.
Just make sure they're as far away from one another as is practical, in terms of geography, et cetera. (We became kind of gunshy about this after the Great Blackout of 2003.)
DNS servers use minimal resources; it's not implausible you can find another use for the VPSes, as well - backup space, secondary MX, et cetera - if you're having a hard time justifying investing $10/mo in, you know, something that's only the most critical and under-rated internet service.