In short
State rules (transfer tax, property tax, attorney at closing, disclosures, foreclosure) are read in each state's statutes and official pages. Costs come from federal datasets: HMDA for loan costs, the Census Bureau for property tax and HOA fees, the Treasury for insurance. Nothing is estimated to fill a gap: a figure Payotte could not read at the source is left out and the page says so.
State rules
Each state page cites the statute or the official page it relies on, with the date it was read. When a rule is set by counties or cities, the page says so instead of giving one state figure. Where the law leaves a cost to negotiation (who pays the transfer tax, for instance), the page gives the customary practice only when an official source states it.
Federal datasets
Calculators
The calculators apply the same rules as the state pages and run in your browser. Each one names its inputs and its sources under the result, and gives the default result without JavaScript. They give general estimates, not a lender's or a lawyer's quote.
Why no professionals are listed
In Canada and in Europe, Payotte selects one professional per trade and per area. In the United States, it does not list any yet. Each state's professions pages say which regulator licenses agents, loan originators, appraisers and inspectors, and where you can check a license yourself.
Corrections
A figure is wrong or out of date? Tell us, with the source if you have it. Pages are corrected and their date changes.