If you could get the fare calculation line from the eticket, that would show the breakdown of the base fare for each segment (I think the taxes/fees might be a bit trickier). I don't believe AA's emailed eTicket receipts show this, and not sure if you can get it online, so your options are to call AA or have an agent at the airport print you the eticket receipt showing the original fare calculation. Here's one from a JFK-SLC r/t (on DL);

Fare Details: JFK DL SLC430.70KE0UA0FJ DL JFK115.35XEVNA0MJ USD546.05END ZP JFKSLC XF JFK4.5SLC4.5

I don't think it's complete with respect to taxes fees (though those are listed separately in their email), but you can see that the JFK-SLC leg had a base fare of $430.70 (fare basis code KE0UA0FJ) and the SLC-JFK leg was $115.35 (fare basis code XEVNA0MJ), for a total base fare of $546.05.