heh... better lube up there bro, 300 gallons of heavy bioload doesn't come cheap, especially if its new. You probably could get away with less than stellar skimmer (new still looking to be in $400 range for a Super Reef Octopus) and combine that with very regular water changes of significant size (you'll end up paying for it with salt still). If it were me, I'd reduce the bioload. 4 fish in a 300 gallon tank poops the same as 4 fish in a 30 gallon tank, it's just you'll notice it less in the 300g tank (and probably could get away with water changes to deal with it).
having a large tank isn't cheap, you're paying for salt, or for equipment, or for electricity, but either way you're paying.