Overread has it about right. Planning and knowing your options is critical,as is knowing what you really need and knowing if something is just a want or a desire. My personal philosophy for over 25 years has been to buy most lenses on the used market,and to allow other people to take the depreciation hit.
As far as used lenses, $500 at one of the Big Five web sites will not go nearly as far as $500 on craigslist or
eBay or in a camera shop located in a smaller,secondary market like Sarasota, or Minneapolis, or Seattle. The biggest web sellers have the absolute highest prices. Real people often let lenses go for things like rent and dental bills and car payments.
Over the years, I've bought a lot of stuff at walk-in retail from two or three one-outlet camera stores. Whenever there is a "new-next-greatest-thing", there is PLENTY of trade-in and consigment gear all over the world. When Nikon intro'd the D100, I bought some of the finest,mint shape manual focus Nikkor lenses I'd seen in 15 years. When Canon intro'd their first pro dlsr, the EOS 1D, the market was flooded with boatloads of good,clean used EF lenses. Lenses are sold off or consigned to finance new purchases, by real people who need cash fast. Small dealers sell this stuff off fast, to raise cash to pay employees and make lease payments. Big Web stores sit on this stuff until they find a sucker willing to pay 30 percent more than walk-in retail. Your $500 cash spent locally could easily be worth $800 nationally.