If the insurance company have said to put the purchase price (which is what 99% of them do for normal vehicles AFAIK), then put that.
The underwriter can't reject your claim based on information the insurer specified. As long as you give them the correct purchase price you're fine. With 99% of normal vehicle policies, the value given is just part of the equation to work out the premium, to confirm it's the right vehicle and to help them tackle fraud cases. The figure you're given in the event of a claim is never guaranteed to be that figure, it could be less or more depending on circumstances.
If you wanted the car insured for a specific figure like some collectors or museums do with high value vehicles, you would need to do a bespoke policy via a broker or someone like Adrian Flux but you would be paying far more per year.