Right but R&D expenses is not the cash spent on R&D, it is a cumulative total of depreciated R&D over the past say 15 years. If Amzn spends $0 on R&D in 2016, that R&D expense will continue (although slowly dwindle) for the next 10 - 15 years depending on the mix of investments. This writing off of investments allows companies that are growing and investing heavily to report current profits under GAAP accounting. But they do not have current GAAP profits, so this is a problem in the future when they can no longer fake growth by undercutting prices.
Are you saying Amazon expenses its R&D, typically this would be depreciated over time, I am estimating Amazon expenses over 12 years on average. Amazon loses money prior to investment in R&D...So without the excessive spending on R&D the PE would still be negative/infinite .
But if you ignore all expenses then the price looks reasonable, as would all companies. Clearly they offer a great product at a low price, but the only reason cash flow is not negative is because of the cash raised by stock options. Who knows when this will end, but this is a $80 - $100 company.