So how cheap is Apple? Don’t be fooled by its $404 billion market cap. With a closing price of $430.47 Friday and $137.1 billion in the bank, Apple may be cheaper now than it’s ever been — and perhaps cheaper than any mega-cap growth stock in history. NOT QUITE: See update below.

I tend to screw up the math in these things, so reader Greg Bates volunteered to calculate Apple’s current price-to-earnings ratio ex cash (6.45) — arguably a better measure of the company’s current valuation than its straight trailing 12-month PE ratio (9.76). (For the record, the S&P 500’s PE ratio is currently 17.27.)

I asked Asymco‘s Horace Dediu if Apple had ever been so cheap. His records don’t go back far enough to know for certain. It’s possible, he says, that Apple’s valuation was lower in, say, the fall of 1997 when the stock was trading for $16.50 a share and there were real questions whether the company could survive.

I have Apple’s share price back to September 1984 and its quarterly EPS back to 1987 (see charts below), but not its PE ratios.

UPDATE: Wolfram Alpha has the answer. Apple’s PE ratio touched 5.76 on Dec. 19, 2000, when the stock closed at $7 a share.  Thanks Josh Staiger for digging that up. See the Wolfram chart below the share prices.