Funny but good question. I am sure that it isn't the ~ $3k Canon/Nikon ask for in additional cost.
Realistically, in comparing the sealing of something like a D700 with a D3, the manufacturing cost difference is negligible, especially if you got rid of the pop-up flash on the D700 which limits their ability to seal it as well. The additional O-rings might be about $10 (likely less). Adding more channels to the case for the O-rings is nearly free, but depending on how intricate the casting is, they may increase the reject rate, which has a cost. Lastly, the design cost needs to be recouped. As long as a manufacturer insists on having a finely incremented product line (many products spanning very small price segments), the extra design cost may be non-trivial. However, reducing nearly redundant models through more modular designs could easily more than offset this (the accessory treadmill, where nearly every new camera has a unique battery and vertical grip).
In the end, when comparing a model like the D700 to the D3, we may be talking about $50 per unit. If we are talking about something like a D3100 vs a D3, then it is likely around $200. I am thinking about internal costing here.
In terms of weight, it is nearly trivial. We are talking about maybe an ounce of O-rings.