Any economics 101 class will tell you the cost of manufacturing a product has relatively little bearing on the price a consumer pays (other than selling for below manufacturing cost).
Please read http://en.wikipedia.org/wiki/Supply_and_demand
Canon is a corporation. A corporation produces products to make a profit. In fact, they produce products to make as much profit as possible. If they feel they could make the most profit by selling the 5D iii for $2500 to many buyers, they would sell it at $2500. If they feel they could make even more profit by selling it for $5000 to fewer buyers, they would sell it for $5000. As such, their economists feel selling it for $3500 will yeild them the greatest overall profit. If their economists are wrong (which they can be), we will see a reduction in price. If they are right, we won't. Competition (from Nikon or any other camera manufacturer) puts pressure on the price by producing more supply of competing products. However, the d800 may not be in direct competition against the 5D3 as each camera seems to be targeted to different consumers.
Ultimately, while general inflation and a weak yen may affect the cost of manufacturing, Canon is more likely to set their price structure based on making the greatest total profit possible. Profit being purchase price minus all associated costs of getting the product to the retailer and boosting demand (manfacturing, shipping, exchange rate, advertising etc).
Going on about exchange rate and inflation while part of the picture, plays a lesser roll in pricing than maximizing profit in my oppinion.
I am not an economist or business owner, so if I am wrong, please feel free to correct me. This is how I understand the economics of pricing.