Today's value of the Louisiana Purchase?
The video has a transcript with the references. The \$0.25B figure is obtained from here (after adjusting for inflation). Unfortunately, the author does not provide a source for the \$1.2B figure. 

However, there are estimates of the value of land elsewhere. For example, here. Their estimates on a map:



These estimates consider the value of land only, removing the value of structures on that land (which, as you say, makes a big difference; see paper). They use hedonic regression methods to separate land and building values (check from page 6 of document). 

I leave to you the task to compute the precise value of the Louisiana Purchase. (you could ask them to send you the data on the map).