As Bruce points out these residuals aren't easy to generalize, but 987s are already starting to turn up on lease trader. 2006s aren't on there yet, but 2005s are (including 2 S models) and the numbers would be similar. The point of my post is to check out what other people are paying on 987 leases to give you the feeling on financing you're looking for.
A large portion of dealer financing on the 987 is done by Porsche Financial Services, so I would get a feel for what they are like plus at least one other lender.
Lease details:
http://www.leasetrader.com/auto/lease/search/2005_Porsche_Boxster_S_Style_92315.xhtml
http://www.leasetrader.com/auto/lease/search/2005_Porsche_Boxster_S_Style_92582.xhtml
Hell, you may even want to assume a lease off of the site, something I considered back when I was looking (ended up buying the box, leasing the merc).
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http://i7.tinypic.com/24ovngk.jpghttp://i7.tinypic.com/24ow0id.jpg
06 987S- Sold
Carrara White / Black / Black/Stone Grey Two-tone
05 987 5-speed - Sold
Midnight Blue Metallic / Metropol Blue / Sand Beige
06 MB SLK350- Lease escapee
Iridium Silver Metallic / Black
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