It's funny that this topic should come up, because I've been thinking along similar lines today with regards to my wife's Civic. A few years ago I switched it from synthetic to conventional oil, partly because I figured it would be cheaper to run conventional. I've been doing 6 month intervals, which usually works out to about 5000 miles. Because I'm using 5w20, the sales aren't as good, so it ends up costing me about $15 for a 4.4L jug of GTX, plus about $5 for the oil filter. So that's about $40 per year. But the sales up here on Pennzoil Platinum this year have been pretty consistent, our Walmart here has had a steady stream of the 4.4L jugs of 5w30 and 10w30 (not 5w20 though) for just $21.88. So I've been thinking that if I switched her car back to synthetic (and back to a 5w30 as well, which is actually what her 2000 Civic's manual originally specified, and I have used a 30 wt oil in that car for most of it's life anyhow) and switched the interval to once a year or about 10,000 miles, then it would only cost me about $27 per year. So it ends up being $13 less per year to run synthetic over conventional! Even if I just went with a conventional 5w30 that had better sales prices, I still wouldn't find it for much less than $10 a jug up here, so I'd still be looking at $30 a year for two conventional oil changes compared to $27 a year for one synthetic oil change.
So I'm definitely wrestling with this idea right now, as I do believe that as a "general rule of thumb" that conventional oils are good for 5000 miles no problem and any of the commonly available synthetics (even if group 3) are good for 10,000 miles.