Tough to measure. A couple data points:

-NY state estimate $150–200M per year lost to real estate transfer tax fraud and evasion. On $3B receipts that is 5-6% breakage.

-Harris County (Houston) found $21M in improper homestead exemptions. On ~$7B receipts that is ~0.3% breakage

-Miami-Dade found $35M on $6.5B for improper homestead exemptions, 0.5% breakage

For comparison, IRS net tax gap is about 12%