I am looking to purchase some used "L" series lenses for my Canon 60D. I am not exactly sure at which point it's no longer feasible to purchase a used lens over new. Surely even the best lenses must depreciate in value over time but I am finding that sellers are pricing their used lenses too close to the the price of a brand new lenses. Area there any good depreciation schedules out there I could use in my negotiations?
No, there is no such thing for any brand.
Lenses are one of the few things that generally increase in value over time. This is more true of high-end lenses. The cheapest lenses such as kit-lenses tend to remain cheap.
Major companies have been known to increase the price of lenses almost across the board. They tend to do it in small increments though but some jumps have been substantials.
Where I would expect a sharp decrease is when a replacement lens comes out (Version 2 or 3 with a new stabilizer or lens-coating for example) or significant change in technology (new mount, added stabilization, new type focus motor, etc).
Based on the last 30 or so lenses I purchased NEW, I can tell you that almost all of them now cost most then when I bought them. The ones I sold USED were ALL sold for more than I paid for, sometimes by only $50 but up to 3X the price too. I'm also keeping some worth about 6X their price now :)
To echo what Itai says I have several lenses that are worth more now used than I paid for them new. Recent price increases for new glass are partially responsible, but the fact remains that lenses hold their value extremely well.
To explain why there are a few relevant factors. Digital SLRs obey Moore's law to an extend in that megapixel counts and processing speeds constantly increase as silicon die sizes decrease making new bodies an attractive prospect diminishing the value of used [digital] bodies.
With lenses performance is mainly due to the quality of the glass elements, even in the presence of technology such as IS, it's the glass that makes a lens.
Unlike automobiles a good lens wont quickly wear out, even though there are moving parts autofocus lenses can carry on shooting for twenty years plus, good manual focus even longer!