It was impressed upon me here and on a couple other forums that once a frame leaves my immediate family, insurance is a must. Not just for MY protection, but for the protection of the person who might be injured riding one of my frames. Those threads that you've already been pointed toward are gold. My own policy now is $1750 per year, and goes up when sales volume goes up (I think the next bracket starts around $20K per year, if my memory serves correctly). I consider it a cost of business, along with the license and raw materials. Letting a frame out without having that insurance is what I would consider foolhardy.

Edit to add: I went through the same agency that e-RICHIE just posted above.