A good piece of advice from an unlikely source (the Globe and Mail) by John Warrillow:
One of the most basic rules of negotiation is to never be the first to name a starting price. If you do, you could leave money on the table, or turn off a prospect before you even start the dance.
Fortunately, there is a formal document you can request from would-be suitors that forces them to put their number - or at least a range - on the table first.
So, what to do if someone is trying to figure out how much you value your company?
There is no need to be the first to name a price. If you find yourself being asked for your number over a quiet lunch, don't feel obligated to react.
Just let your suitor know you'd be happy to review an IOI or LOI. A serious professional buyer will know what you're looking for, and will be happy to oblige.