Indeed, Apple have said 18ct Yellow and Rose gold options.
The only thing we have no idea on is the wall thickness of the case (or shell) of the watch.
I have speculated, due to scale, and needing as much internal space as possible for the Electronics, Screen, Battery, Sensors etc, that a wall thickness of 1mm would not be unrealistic.
1mm does not sound a lot I know, but at the scale of the entire device, it does not visually look bad.
If it was to be 1mm wall thickness, and we just don't know until one is stripped down and measured/weighed.
Then we are (with a little bit of educated guessing in internal areas) looking at approx. $1200 in material value of the actual gold case.
As the internals are the same as the $349 watch, then lets say we add $200 for just the internals.
And, just for the hell of it, let's add on another $100 for the ceramic and sapphire front and backs (though given their size, they won't be THAT more expensive than the sports model) but we'll round it to $100 anyway.
That would (and it's an educated guess I know) place the Gold edition models at $1500 plus the cost of whatever strap you wish to select.
How much more than this figure they cost to the customer, is going to depend on how much "Fashion Profit / Mark-up" Apple wish to place on them to elevate them to a position where they are SEEN to be expensive and unattainable to the mass public, and something to look up to, or feel special about if you do own one (show off with basically)
Given the numbers I have shown here, I struggle with the $3000 to $5000 speculation, unless the gold case is MUCH thicker than my guestimated 1mm wall thickness.
If anyone thinks 1mm is crazy thin I can supply cross sections showing how that would look like to scale.