Condition. Condition. Condition.
And no two are the same so it's a wide-open wallet for the best examples.
Recently-completed conversions are rarely on the market and older conversions range from "experienced" hard-workers to worn out basket-cases and disassembled projects. If you find a truly nice converted airplane with low engine times and clean airframe with useful avionics that don't appear to have been installed from the results of an online-auction, then the price is most often influenced by the situation of the seller. Inactive/Inherited airplanes are usually available at "value"-prices, but they usually resemble restoration-projects. Finding one with good engine times, all the logs, and nice cosmetics and equipment is going to bring a fine price. Since "original" 170-B's in better condition bring $40K-$50K, it's not surprising that equally-fine conversions cost another $15-20K as Bruce suggests.
Blue4 wrote: ... but what's a non-insulting offering price? -Scott
There are owners who swap airplanes as often as they eat out....and there are those who keep and treasure them because they have personal-effort and/or memories invested in them. It's difficult to insult the former and hard not to the latter.
I'm never insulted when someone recognizes the true replacement value of my airplane, even tho' it's not for sale. I'd think the same would hold true for conversion-owners.
(While discussing toy-prices.... have you visited a gun-show recently? Good, classic firearms are getting very difficult to find for-sale at any price...but when they are, the prices have resembled the gasoline-prices of last spring. It seems like every time gasoline spikes up... everyone figures that everything else should follow. A pre-64 Winchester model '94 used to be considered pricey at $250 only ten years ago... and now they are priced at $1100-$1300.

I don't agree with those who say that aircraft values are falling due to the economy. There are some types of airplanes that are priced very low these days....(such as Grumman-American Yankees/Tigers and Traumahawks...) but the classicly-attractive models will hold their own and escalate, in my opinion, because they are still scarce and getting scarcer.)