what seems to make my mpg readout drop is the idling at stop lights and such. When on the highway with cruise set to about 80 mph, I reset the mpg/avg speed settings so it can start fresh, and I seem to sit around the 23-24 window. Mine is perm. 4x4, which I'm not crazy about. Around town, it drops down to about 18.5-19.3 on a gentle foot. 16.2-17.0 on a heavy foot. But, in the V8, you only need a heavy foot if you need to haul butt, and you're above the speed limit by a good bit before you know it.
In terms of MPG and cost of operation differnces in gas, I'm sure the V8 costs more on an annual basis, it's just plain logic that it would. But, the question you need to answer for yourself in deciding, is having the V8 under your foot worth probably an extra few hundred dollars a year? For me, it's a no brainer, well worth having that kick in the pants ready and willing when I need it/or want it. Probably costs me an extra $300-$400 bucks a year in gas over the v6 model. But, I'm ok with that. Plus, as mentioned earlier, you have better odds when buying used that a V8 hasn't been worked as hard as a V6. Simply due to the V8 not needing to rev has high to pull the car. Cost increases of gas, can be offset by less repairs due to more engine wear incurred.
The other fact that most driver types that would chose to abuse an engine (teenagers-mid 20's) simply prefer the insurance costs of a V6, instead of a V8. So, better odds of getting one driven by an adult. Mine, at 60k now, is still on it's original michelin tires, rotors, and rear pads. The front pads were replaced by the dealership when I purchased it. The only thing that tips off a driver of the miles on mine, is the odometer. Everything else about it suggests maybe 20k miles.
The V8 is so good, I'll tell you a true story on my old LS. I bought it from a Lexus dealership back in 04', with 86k miles. One owner, locally owned, normal maintenance done. The dealership used the car for about 5k as a "tester" for people thinking about buying a new LS430. They put a black piece of tap over the odometer and told people to take it for a test drive. When they came back, they asked the potential customer to estimate the age and mileage of the car. Nearly every single one guessed it was about two years old with around 20k miles. It was 9 years old, with 4 times the mileage. They sold more new LS430's that way than any other. To this day, chances are you can find an older LS400 sitting at a lexus dealership being used for the same thing. I was on the fence about buying it, until the salesman told me that story. A few minutes later, a customer walking out of the dealership to get into his new 04' black 430 saw me checking out the 95', and said "I bought this one because of that one" and confirmed the salesman story. Best car I had ever owned. Drove it to Key West, Pennsylvania, NYC, KC, Birmingham and countless trips down to the coast, with out a single problem.