Depends what you mean. If it’s literally what does a Westfield with an S2000 engine in cost, then about the lowest I can remember off the top of my head was around £8/9000 years ago, for one of the early S2000 engine transplants into a modified Westfield that someone had done. However, there are very, very few of these cars around, as it was always regarded as one of the most tricky/expensive engine swaps to do. And of the conversions done, I suspect most were done as quite high spec competition setups, so not cheap, as and when they occasionally come up for sale.
However, the “proper” factory engined car, the Mega S2000 is only three and a bit years old as a model, and was a premium model from launch, again, while it’s been popular, not many come up for sale. The range of prices I’ve seen seem to be typically high teens to low twenties, depending on spec, age, condition etc.
The fact that the S2000 car itself has now started increasing in value as a modern classic means that we’ve also seen the supply of engines and transmissions for a few thousand or so, now start to be high threes and even four thousand plus. This will ultimately also have something of an effect on Westfield Mega S2000’s values too.