When I price out BOS-INV in Club taxes come out to roughly $550 one-way. When I price out INV-BOS in Club, taxes come to roughly $285 one-way.

But when I try to book return, BA tries to charge me around $1,100 in taxes. Same flights, same dates. Why?

Ordinarily, I'd book each way separately. But I am trying to book using the 2-4-1 companion voucher, so I have to book the trip as a return. But instead of $1,670, it tries to charge me a total of about $2,200.

Any suggestions?

Thanks in advance.