I think there's a higher chance that earnings are lower in the next 5,10 years. If 60% chance earnings are 20% lower than 2012 (44*0.8) and 40% of the chance earnings are same (44), normalized earnings would be (44*0.8*6+44*4)/10 ~ $39/share. 15x might be ok if you look at this number as a coupon for a risk free bond, so $585/share could be fair. But then the smart buyers would wanna buy close to mid 300s for a margin of safety. I think Apple would buyback at $350-$380.
I hope I'm way off and the conditions are way better than I expect. Maybe earnings never go 20% lower than 2012 and the odds are not what I expect. I could be too pessimistic.
Keep in mind that if earnings really did level out at $39 per share, that Apple could keep its $145 billion cash pile and put out yearly dividends of $39 per share. After ten years an Apple shareholder would have (a) the same Apple share worth something pretty good and (b) $390 less taxes (under current rates leaving you $312). So you can buy at $395, get $312 in cash and still have the same share which you can probably sell to the market for something higher than $395. Every investor in the world would love to get an investment like that.
The current shares are only correctly priced if Apple's revenue drops significantly over the next several years. With reports coming in like Verizon sold 25% more iPhone's in 1Q2013 compared to 1Q2012 and T-Mobile selling iPhones for the first time ever, that supports that this year isn't going to be a big role back in revenue.