We were going to have my nephew sit in my sister's lap for our trip to WDW in January. When we were first looking to cost out our airfare, the taxes for him were around 12.65 on Westjet. My understanding is that it is the US Immigration Fee (7.00) and the Passenger Facility Fee for Orlando (4.50), both of which were in US dollars, so it came to 12.65.
We eventually decided to get him a seat (and pay about $120 in taxes).