The pre-IPO bids for Facebook at SecondMarket were substantially higher than the original IPO (first-day trading price), less than a week before private trading was halted. Even the private market has no idea what non-public shares are worth. IMO, ask for an increase in salary, or tie the value of the stock to a conditional bonus payout. Keep in mind that even if the company goes public and the stock does well, you probably won't be able to sell for 12-18 months due to lockups, so the IPO price is less relevant to your compensation than the long-term stock price. If you are smart (and have the cash), a 83(b) election can be hugely beneficial for your taxes... unless the price at which you do the election is far above the eventual market price of the stock, in which case you are out quite a bit of money.

Edit: If the company truly feels that their stock can be substituted for salary, then it shouldn't have more than a 12-month cliff for vesting.