Well, there's more than a simple answer here.
First, there are basically three different eras of Fisher speakers.
Third and last, anything from the end of the Fisher brand run, during the 1980's Sanyo era, has no collector value whatsoever. This includes any speakers that start with ST or STV.
Prior to this period, there are some very high-quality speakers, and some very ordinary speakers. The various model numbers don't always tell you everything you need to know, so you can usually rely on the old standard tests: If it's heavy, and it has real walnut veneer, it's worth a try.
The bookshelf sized ones pretty much follow the above guidelines. The floor model "Consolette" line are all very good speakers. This would include any speaker from the XP-10 to the XP-18.
Here may be one of the main reasons vintage Fisher speakers are not sought after, especially if you compare the sound to current speakers. The legend is that Avery Fisher insisted on personally "voicing" all of the speakers. Apparently his personal preference was a very warm sound, one that today would be considered to have very rolled off highs. At the time it drove his engineers nuts. So, if you're willing to swap out the tweeters for something more current, ideally with a silk dome tweeter to at least stay faithful to the era, you might find a very lovely sound. And, at a very low price!