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As an example, look at the key lock S&W revolvers. The pre-lock models often sell for higher prices than the new ones do. The immediate recall on the new Pythons didn't help their rep either.
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That's not really a fair example. Most Pre-lock S&W are actually superior to modern S&W's, its not just about the lock or age.
By all accounts by those who know, the new Pythons are actually superior to the old ones. No, the issues with the first run don't help (even though that seems to be the norm these days for all manufacturers) but it really comes down to why you are buying it?
Are you a collector? Then sure the old ones will probably always hold a premium over the new ones, especially the royal blue models because Colt doesn't have a blued steel version of the new ones and those blued steel pythons are gorgeous but if you are a shooter looking for something you will actually use, I would absolutely get a new one even if they cost the same $.
OP - hard to say without more information but assuming 80-90% condition, no box, etc., I'd say ~$1500-$2000 tops. Closer to $2k if the condition is in the 90% range and has the original wood grips in equal condition. $1500 or less with more wear and plastic and/or non-factory grips.