In a perfect world, and if you consider Bitcoin a true currency, then the only price that matters is the amount you value your work in relation to what you can buy and do with the currency. But of course, since you currently can't use Bitcoin to pay the necessities of business like rent, electric bill and phone bills, the problem you mention is real.
The only real advice I can give is to watch the market for points of stabilization and price your coins according to what you believe the real market value of a coin is going to be at the time you will need to convert them to pay your bills. Also, you should carry enough cash reserves in a more liquid currency to cover you in times when coins are lower than expected. In those months, use your cash reserves. When coins are high, sell extra to replenish cash reserves.
A safe bet right now IMO would be to value a coin at about $10. That seems to be a recent plateau that makes sense and with adequate cash reserves, you should be able to float the spikes and dips without a problem. Of course, keep your eye on the exchanges for movements and if there seems to be a longer plateau and your reserves are getting low, then you will probably have to revalue your work.
4For many years here in israel rent was quoted in dollars but paid in shekels at the current day's exchange rate. So you could quote the price in dollars and ask to be paid in bitcoin at the rate on some well known exchange – Zachary K – 2012-09-07T08:37:38.510