Probably more than double it. The UK has a 500k ish (in pounds) inheritance tax threshold and a similar tax rate(40%) and collects 5 billion a year compared to the U.S. just shy of 20 billion. Considering the U.S. economy is about 7 times the size of the UK and has similar individual wealth levels you can ballpark that if the U.S. dropped to that threshold it would at least double.Over time though it would skyrocket because of urban area housing price appreciation.