[quant]

This should be very easily implemented cause the search goes through the index anyway, it could be a first (system) calculated attribute in UR that would be automatically updated when the search is performed.

My suggestion for simple formula for relevance attribute:

20*(number of found (unique) keywords out of all searched)
+ 10*(number of searched (unique) keywords in the title)
+ 1*(sum of occurrences of all found keywords).

The explanation is simple:
If the item has certain keyword 10 times, it is the same as if it was just once, but in the title. And it would need to have say 20 of occurrences of some keywords to be at least as relevant as another item that contains one more (but unique) keyword. Or it would need to have two keywords in the title to be as relevant as another item that has one more unique keyword ...

what do you think?