Hagerty's value estimates seem to be a bit inflated! Not surprising for an insurance company that makes more money from premiums on higher valued cars.
Most TR6s in good to excellent condition (meaning little to no rust!) struggle to get out of the $20Ks range. Many cars in the $30Ks range are listed by dealers or the occasional very optimistic owner. On BAT, only 8 cars out of many, many over the past few years went for over $50K, and only 4 sold for over $40K--all of these within the last 2 years or so. Does this mean that the market valuation for TR6s is going up? Not so sure. Each of the cars sold in the $40K+ range were exquisite examples--very low mileage, over-the-top restorations, extraordinarily well built peformance upgrades, etc., etc. These were likely headed for collections, not for for your average DIY TR6 owner/driver.
And that really is the point. If you want to really enjoy your TR6, you have to drive it, which means that parts break or malfunction and have to be fixed. Fortunately, companies like Moss and The Roadster Factory continue to supply good quality parts, and for those interested in more performance, Good Performance Parts is simply THE place!! And while these companies supply parts (and expertise) as the number of British-speciality mechanics likely diminish each year, the TR6 remains primarily a DIY car--no longer manufactured, no dealerships, etc. Unlike BMWs, Porsches, etc. which continue to have dealerships and speciality shops where the well-healed owner can take his/her classic for repairs and upgrades.
So I would caution current TR6 owners that if you really want to sell your beloved 6 in today's market, and it is a good to excellent example, keep the price in the $20K range, or into the $30Ks if there's something special that sets it apart. But in the long run, buying a $15K car and then paying someone to restore or upgrade it (which can easily cost $20-$30K on up) will still end up with a car that you can sell for $20K-$30K. Sadly, I don't see any long run appreciation for our TR6s. Collectors are more likely to spend thousands on a Porsche, BMW or other classic car than on a TR6.
So buy and enjoy; repair as needed. Keep 'em on the road!