In TurboTax Online, Cash Received is entered in the Like/Kind Interview (screenshot).

This shows on Form 8824, Summary Smart Worksheet, Line L.

In TurboTax Desktop, you can make entries directly on the form, which would make your 'what if' scenarios easier. 

Click this link for more info on 1031 Exchange with Boot.

I know where to manually enter "cash received", but I think TurboTax should automatically calculate this value as the difference between the net higher value property given up and the value of the lower priced replacement property.  This would eliminate the possibility of inadvertently entering an incorrect value or leaving one in that relates to another scenario I have built.  It also would quickly show how much cash I'd be receiving so I can fine-tune my target replacement property value.