I had a quick look on eBay and a 748 with under 5K in what looked to be clean & tidy cond, sold at a dealer for £5500. So I'd guess the same one would fetch £5000 or less privately imo. Trade in finance & short warranty are important to many buyers. Which leads me to think standard -beautiful- 748's advertised for £7K are staying in the sellers lounge for the forseeable.
You are 100% right about the “conviction”, but to be fair, I think the track was the intended destination for the R versions.