I agree there's no reliable way to value a startup, but that doesn't mean you should throw your hands up and say they're worth nothing. In lieu of any reliable method, the best estimate you have available to you is the company's last private valuation. Though there is wild variation in outcome, on average, VC's do a decent job pricing investments.
If the best estimate of the value of the stock is it's last valuation, that means current options, which are priced at last valuation have a paper value of $0. There's certainly a time-value, but that's hard to compute, given the most popular model uses a volatility component which you'll have no data for.
If you have options from before the current valuation, or if you were offered RSUs, or underpriced options, you could use the valuation as a guess of the value, sure.
To my knowledge, almost all startup options have a strike price that is close to the last 409A valuation, which is typically an order of magnitude less than the valuation given to the company at its last private financing. Receiving options with a strike price close to the last private valuation would be highly unusual, but if it did occur, I'd advise people to value those at $0.
When you say "typically", how many data points do you have?
In my experience, an order of magnitude just happens when the company is at a very early stage (e.g. post seed). By the time the company is at Series C/D, the 409A valuation is typically between 1/2 and 1/3 of the last preferred round, or higher.
