Transfer duty, stamp duty, whatever the local name is, almost always runs on a marginal bracket scale. Each slice of the price is taxed at its own rate, and crossing a threshold changes the rate on the slice above it and on nothing else.
duty = sum over brackets of (amount falling in that bracket × its rate)
The bracket scale calculator takes the scale itself as an editable list of threshold and rate pairs, because every jurisdiction has its own and they change with budgets. The figures below use the illustrative scale the page starts with rather than any particular state or country.
What the brackets do
On that scale a $750,000 purchase attracts $18,250 of duty. The effective rate across the whole price is 2.43 per cent, while the marginal rate at that price is 4.5 per cent. Those two numbers are nearly a factor of two apart, and the smaller one is the one that describes what you paid.
At $2,000,000 the duty is $84,500. The effective rate is 4.23 per cent and the marginal rate is 5.5 per cent. Notice that the effective rate has risen but still sits well below the top marginal rate, and it always will, because the lower slices keep being taxed at the lower rates no matter how large the price gets.
Thresholds are not cliffs
Here is the test that settles it. On this scale, a purchase of $999,999 attracts $29,499.96 of duty. A purchase of $1,000,001, two dollars more and over the million dollar threshold, attracts $29,500.06. The extra two dollars of price cost ten cents of duty.
That is what a marginal scale does, and it is the opposite of the common fear that crossing a round number retriggers the whole bill at a higher rate. Negotiating a price from $1,005,000 down to $995,000 saves you duty on the ten thousand dollars, at the rate for that slice, and nothing more dramatic than that.
The cliffs are in the concessions
Real discontinuities do exist in property duty, and they are almost never in the bracket scale. They are in the concessions: first home buyer exemptions, owner-occupier discounts and thresholds for foreign purchaser surcharges. Those are frequently written as all or nothing, so a dollar over the cut-off can cost many thousands.
That is why the calculator has a separate concession field rather than trying to build eligibility into the scale. Eligibility rules are the most jurisdiction-specific and most frequently changed part of the whole system, and a page that guessed at them would be misleading rather than merely out of date.
Duty is usually the largest single cost of buying, so it feeds straight into what the sale has to fetch to come out even. The same marginal logic runs through income tax, and the post on adding and removing tax covers the related trap of working a percentage backwards.