I value comabies based on a DCF model, but starting with a valuation with a cash flow yield related to the uncertainty would be a great start. So for a quite uncertain business 2.5 times yearly cash flow would give it a 40% yield on cash flow.
Yup. Net-present-value of expected future cash flow + current assets, same as any other business. The trick is estimating future cash flows of course!
So, take what Mt. Gox is currently earning, and their growth rate to date, modify according to your own subjective assessment of how the bitcoin market will shape up over time, and do the math. The range will probably between about $0 and $500,000,000,000. That helps, right?
Bottom line: it's hard enough valuing traditional businesses with relatively stable cash flows. With bitcoin, there's far too much subjectivity to make a good hard case as to long-term valuation, so I like wachtwoord's standard conservative 2.5x cash flow approach.