It looks like Apple will launch the next iPhone on Tuesday, Sept. 10. This means that the new model (or models) could be available in showrooms as early as Friday, Sept. 13.

If you’re a current iPhone owner and wish to upgrade, now is likely the best time to sell your handset on the secondary market.

According to NextWorth, the residual values of older models usually begins to fall 3-4 weeks before the announcement of the new iPhone. In other words, prices are likely to begin dropping soon.

According to Jeff Trachsel, NextWorth’s CMO:

There is a predictable 15-20 percent value decline seen across all older iPhone models in the six-week period surrounding the new iPhone launch.

To maximize the time people have to send in their old phones, NextWorth has extended its quote window from 21 days to 30 days. This allows customers to lock in peak pricing now, but keep their current phone for 30 days.

Here is a sampling of what NextWorth is paying for old iPhones as of today, Aug. 14:

  • iPhone 5 16GB White AT&T – $314
  • iPhone 5 16GB Black Verizon – $314
  • iPhone 5 32GB Black AT&T – $316
  • iPhone 5 32GB White Verizon – $315
  • iPhone 5 64GB White AT&T – $410
  • iPhone 5 64GB Black Verizon – $370
  • iPhone 4S 16GB Black AT&T – $210
  • iPhone 4S 32GB White AT&T – $214
  • iPhone 4S 32GB Black Verizon – $171
  • iPhone 4S 64GB Black AT&T – $245
  • iPhone 4S 64GB Black Verizon – $205
  • iPhone 4S 16GB White Sprint – $170
  • iPhone 4S 32GB Black Sprint – $171
  • iPhone 4 8GB White AT&T – $135
  • iPhone 4 16GB White AT&T – $137
  • iPhone 4 32GB Black AT&T – $140
  • iPhone 4 32GB Black Verizon – $73
  • iPhone 4 32GB White AT&T – $140
  • iPhone 3GS 32GB Black – $33

To see what NextWorth will pay for your handset, visit their official website to ask for a quote.

We’ll continue following the secondary prices for old iPhones in the days and weeks ahead. In addition, we’ll be publishing our annual iPhone Trade-In Guide during the week of Sept. 2.