If it is a genuine ventile one then maybe around £60 if its in fair condition. I doubt it is ventile though if its genuine. You do get very strange smocks arriving every now and then which seem to be the genuine article- often from various trials.
I strongly suspect it's a standard arctic smock- on issue to the Royal Marines a few years back- as is mentioned below, genrally the same as a windproof smock (known as a SAS smock), but with a huge wired redesigned hood (not really necesary for standard use) and rank tabs on the front and back.
Good smock (as is the SAS smock), dries quickly, and is windproof. Due to being good quality tightly woven cotton, you do get a little of the ventile effect- i.e. the fibres swell when wet to seal the fabric better- but anything more than a light shower and the water will penetrate.
You can get reasonable used SAS smocks from the likes of ebay for £10-£20.