I know a guy in my marina bought a 300 hp merc 4 stroke and he says the thing sips fuel. He said he gets close to 3 mpg with it 
. The motor cost him around 28k and his old motor only got 1.25 mpg. so for that and as much as he uses it it would take 4 years too offset which a 4 year old engine still has a looonnngg life 
A 4 year old engine has a ton of life left. For me, since my engines are really old, 1995 2strokes, I am buring about, unofficially, 25 gallons per hour each, 50 total. Any newer engine, 2 or 4 stoke should bury that. I got a price last year for Mercs, twin 225's @ $32,000, boat show price including rigging. So, all for all, not to bad. If I get a few buck for my engines, it might not be that bad. I would probably use the boat even more if i didn't fear the cost of fuel.
