Determining "what's a piece of this thing worth to me?" requires a bit more noodling than does outright ownership. Conventional wisdom imposes a discount on any asset in which the owners absolute rights are impinged by others.
This principle obviously applies to co-owned airplanes, since the other owners have a voice equal to or greater than that of a prospective buyer. Using this logic, a buyer might seek to impose a haircut of some amount, since he will be required to share flying time, scheduling, admin and decision-making with others. If you assume that airplane interests will trade within a certain range that approximates a percentage of the FMV of the entire plane, you might then expect to see a co-owned piece trade at the low end of the range.
With that in mind, I encourage clients who are considering co-ownership to perpare an own/op budget for some reasonable ownership period in order to quantify the cost differences between full ownerhip and their proposed co-ownership. Using Weilke's example of a $2,000 cost swing, it's obvious to a tenant in my hangar complex that he would save ~$5,000 in rent and insurance alone during the first year of ownership, without consideration of maintenance and other costs. Assuming the new guy has reasonable access to the plane when he needs or wants to fly it, the bargain element in co-ownership represents an incredible deal, and negates some of the haircut logic that might otherwise be imposed.
FWIW, I don't get particularly excited about any price that is within a reasonable range of the indicated market value, simply because the down-stream benefits are so good and the other owners are already in place.
Buying a piece of a plane is more difficult due to:
1. The due diligence required to understand the structure under which the airplane has been owned.
2. The need to fully understand (and perhaps modify) the current JOA or to implement a soundly-constructed version that includes the hard parts that are not immediate but can quickly become problematic (wind-down, dissolution, divorce, death or disability, financial non-performance, co-owner buy-sell, sale of the asset) and which many owners defer or simply ignore.