Best way is to trawl autotrader for VR6s using a radius of 40 miles from your postcode. You can do it easily on their website. Take a look at what other people are asking for theirs and you'll get a good idea.
My gut feel is that if it's a nice example, somewhere round the £2,750 - £3,250 mark sounds right but it depends on how nice it is and where you live. Maybe a little over that for the right colour and spec.
Good luck if you decide to sell. Mk3s are still sought after - I put my 1998 Mk3 8v GTi up for sale just before Xmas and sold it within 24 hours. Loads of calls too - could have sold it 6 times over!