Two numbers matter more than the total. The first is the allowance for anything we cannot see until demolition starts, because in a house from the eighties that is where the surprises live. The second is what a change costs once the work is underway: moving a sink after the plumbing is roughed in is a different job than moving it on paper.
Ask for both in writing. A contractor who will not put a number on either is telling you something.

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Where the money actually goes

That is why we open a wall before quoting a firm price whenever the house is older than about thirty years. It is a small inconvenience up front, and it is the difference between a change order halfway through and a surprise you did not budget for.

The other half of the answer is scope. A kitchen where the sink stays where it is costs meaningfully less than one where it moves across the room, because the plumbing follows it. Ask any contractor to price both versions and you will learn more about your budget than any online estimate will tell you.