What is the worth of a 1962 porsche 356b carrera twin grill?
 

It's a valuable car (assuming we're talking about a genuine 356 Carrera... the regular 356 is still valuable, but not close to the Carrera). Though a few key variables that you don't mention. First, is this a Cabriolet or a Coupe? The Coupe is a rare car, but the Cabriolet even more so (just over 30 of them were ever made). The next issue is if the engine numbers match the chassis. After that we get to the condition of the car: first is if the chassis in good condition and rust free (and if so, is the paint original and in likewise good condition). Second, is the engine running (and running well). Then, finally down to issues like the interior, other original parts, and lastly things like trim.

These cars do not come up for sale often, but here's what I've seen. An all original coupe with matching numbers and such in excellent condition would probably command about as much as $300k. A similar coupe, though restored and in Concours condition would probably be about $250k. Even if it's in otherwise terrible condition, if the numbers match and the shell is in solid condition, the car could still fetch close to $200k.

If it's a Cabriolet: It's almost a guess as these are extremely rare! I'm not aware of any changing hands in recent history, but given the recent rise of 356 values, it wouldn't be reasonable to expect that such a car, even in terrible condition but with matching numbers and a shell that was at least repairable, would still command $500k or more. The upper end could be double that for an excellent example. Mb Star Compact C4


If the numbers do not match, values will still be pretty good as long as the engine is an actual Carrera four cam (and potentially little hit in value if you can trace the history of that engine and connect it as a factory provided replacement).