SELECT ... WHERE fti_query ORDER BY numeric_col LIMIT x - problem

I have toyed around with KNN a little and I am pretty impressed when it comes to the results we have seen in the GIS world.
Given the infrastructure we have at the moment I wonder if KNN can help to speedup queries like that:

SELECT ... WHERE fti_query ORDER BY numeric_col LIMIT x

The use case is fairly simple: Give me all products matching a certain tsquery and order those products by price or so to show the top 10.
KNN is supposed to gives sorted output which works perfectly for points and so on but can there theoretically be a sensible / reliable distance function for a (tsvector / numeric) combination? Some first testing gave me some "interesting" output .
If there is no way of defining a reasonable distance function performance is screwed up if fti_query returns a large list (it requires a complete sort of all prices then).

does anybody have some useful input here?

many thanks,

hans

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