Q:

In (the) Six Figures

Suppose I am talking about the cost of something that is in the six-figure range:  

A complete remodel has a cost in the hundreds thousands of dollars.  
A complete remodel has a cost in six figures.  
A complete remodel has a cost in the six figures.  

Should it be "in six figures" or "in the six figures"?

A:

The first sentence sounds wrong to me. If I were to say that in everyday speech, I would use cost as a verb rather than a noun, and say

A complete remodel costs hundreds of thousands of dollars.

If you want cost to be a noun, you would either say "A cost of some value" or "A cost in the range of values. Now, in this case "Hundreds of thousands of dollars" is not a specific value, but it is still some value. Just like you can say "It will take me hours to finish!", or "Light travels millions of meters per second." and you're talking about an value not a range. A range would be if you say "It will take between 4 and 8 hours to finish!"
That means you could say:

A complete remodel has a cost of hundreds of thousands of dollars.
  (Vague amount)

Or

A complete remodel has a cost in the hundred thousand dollar range. (Anywhere from 100,000 to 999,999 dollars)

With sentences 2 and 3, neither of them are correct, but they both could be correct with a minor change.
Sentence 2 should be:

A complete remodel has a cost of six figures.

and sentence 3 should be:

A complete remodel has a cost in the six figure range.

