Not really sure where your getting your inflation data. According to this page: http://www.usinflationcalculator.com/,
based on CPI inflation, $2999 in 2007
is about the same as $3278 today
, due to a change in inflation of 9.3%.
You could try to factor in commodities inflation, however those are fairly volatile markets, and month-by-month inflation or deflation in commodities (which can indeed be extreme over the short term) are not nearly as severe over the long term.
I think the 5D III is a bit over priced regardless. Based on CPI inflation, its about $300 more than it should be if we just accounted for inflation. It certainly has a major boost to some key features, like AF, but I'm not sure that really matters in the face of highly competitive pricing from Nikon on their D800, placing it at $3000. Inflation of the D700 starting price of $2999 would indicate that an equivalent pricing today of about $3157. (So Nikon is underselling the D800 on an inflation-normalized price curve.)
Either Canon really thinks they have a whopper of a camera housed in the 5D III and it will sell like hotcakes regardless the price, or they will be dropping some huge rebates in a few months when sales don't pick up like they expect it to. (Personally, with the Olympics just around the corner, I think the 5D III will sell like hotcakes for a while, and rebates will drop a few months after.) Once sales drop off, Canon will have to get competitive on price, and I figure it will settle around $2999...in line with the D800 (and most peoples expectations of a fair price.)
(BTW, if $2999 in 2007 equaled $4200 today, that would mean we had 40% inflation
in about four years. There is NO WAY we've had that much inflation in four years. Its been about 1/4 that much on a normalized basis.)