There doesn't seem to be a clear value or formula.
For example if the .com was owned and forwarded to a site or certainly not for sale or going to be developed any time soon I would expect that to push the value of a .net up a bit higher than normal.
One person may say .net worth 10% of .com someone else says 3-5% of the .com value.
.co.uk doesn't compare as they are their own market imo, UK domains seem to be gathering trust in the marketplace and value, I am sure quite a few companies that are only UK specific would prefer the .co.uk, also a few more investers with money are looking into the UK market which should help reseller prices.