If I am looking for a particular lens, I go to *Bay and watch several auctions for that or similar items.
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Go to keh.com and check their prices
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Check out the Pentax and Third-Party lens databases on this site.
Prices are generally all over the place depending on rarity, reputation, and compatibility with current generation Pentax DSLR bodies.
The ranking ($$$$ to $)
- Pentax LTD, *, and Pentax/non-Pentax cult classics
- Pentax FA/DA
- Pentax A, certain Takumar cult classics,
- Non-Pentax name-brand third party (KA-type mount), certain Off-brand cult classics (regardless of mount)
- M-42 SMC Takumar
- Pentax K, M-42 Super Takumar
- Pentax M
- Off-brand third party regardless of mount, Non-SMC K-Mount Takumar