Based upon optimistic earnings estimates for 2014 ($7 a share?), a 20 forward PE (twice Apple PE) might say a price of $140 for Amazon is appropriate. But future earnings are written in sand. Any rise in shipping costs, or economic slowdown, could reset the estimates. If earnings only come in at $4 in 2014, a price of $80 might be appropriate.
JPM recently revised 2014 EPS estimate to $2.83 and lowered target to $300/share. With future earnings written in sand as you say, it is a Bizzaro world where it is normal to have a P/E ratio on 2 year forward earnings of 100+. I get that Amazon is the market leader, growing market share in a growing market and that it is redefining retail, and "disrupting" other markets, but none of that is guaranteed to last to 2020 and beyond, as most models require for AMZN to be fairly valued at today's price.
Even if Amazon eventually reaches its promise, my guess is that at some point along the way doubt will return along with a more realistic valuation. I'll take a stab - fair price 30x $3 EPS = $90.