A settled great scientist generates 6

each turn. Let's say you have a 200% multiplier in your science city, so that becomes 18

. Usually bulbing a tech gives you, what, 1400

? So settling a great scientist will take 78 turns to pay back speaking just in terms of

. That's an awfully long time, IMO, to the point that I'd think it'd be better to chop some turns off of a tech you need to research, or go for a golden age, rather than settle. It's difficult to quantify the value of getting a tech three turns earlier than you otherwise would, but CIV is all about snowball effects -- say, getting Biology early means better farms means more food to run scientist specialists means faster tech, et cetera.
Of course the settled GS also gives you 1

. But you'll be settling them in your science city, which at least in my games generally has lots of food to run scientist specialists, not lots of hammers to do production.