Between Ã‚Â£2 and Ã‚Â£5 Thousand, condition dependent. Some later examples of P4's than this one (1955) have reached Ã‚Â£6,000+ on the day, it depends who is seeking one at the time.
I would think the example on eBay is worth well less than the asking price, considering the restoration required.
Many parts would surely need to be made from scratch, and having been involved in such endeavours, you need both deep and disregarding pockets. The rewards, (in my experience) are sometimes depressing financially, but spiritually very uplifting.
If you can afford to restore and hang on, the benefits can be spectacular. Having said that, I don't think any Rover would presently fall into that category. More a labour of love, re-creation and then enjoyment of ownership I would imagine.