V8s have a few extra servicing costs, more bits to go wrong, and guzzle fuel, but I wouldn't say they are inherently that much more expensive than any other car. Look at the LS400 (XF10 / XF20), they have V8s, but are one of the most reliable cars ever made, so costs are low. The cars that will bankrupt you are the unreliable ones with big bills.
Much like a Corvette, the VXR8 isn't fancy, but to some that is part of the allure, even if the interior sucks. They are simple, powerful, and easy to work on / mod, plus there are plenty of specialists around for the more complex jobs.
However, the biggest advantage to the VXR8 is the LS engine. It just opens up infinite possibilities and has immense capability. If you want to stay stock or just do some basic mods (intake / exhaust), then the IS-F is probably the better choice, but the VXR8 will get you those big numbers for very little money (comparatively).