Market value is market value. If it is worth £10k today and £8k in 1000hrs thats normal depreciation. Put a new engine in and the value will not rise back up to £10k despite what you may hope. The engine fund is just a way of buying your way free of a massive bill when the donk fails. Think of it as HP for liability. This is nothing to do with the value of the airframe.
I was only involved in a group once I bought a share for £8k. The engine fund stood at about £10k. I flew a few hundred hours in it and paid the wet rate. £20ph went into the engine fund to offset my libaility for the hours I had "worn" on the engine. When I sold the share the buyer bought my share and continued to pay into the engine fund offsetting there own liablity. Effectivly they were buying into the group knowing they did not have to cover my wear and tear on the engine but knowing that they were buying into possible liablity if the engine failed early.