a few years ago (post Warnock) there was a quote from someone at the club (can't remember who) possibly when we were signing the likes of wintle and Ng that we would be avoiding paying a transfer fee for older players who would have no residual value unless in exceptional circumstances. Older players should be on a free and a fee should only be paid for younger up and coming players who have the chance to increase in value.
to me this sounds eminently sensible, and as far as I can tell we've largely stuck with it for the last couple of seasons (although possibly only because we had an embargo)