Fair question. I did get somewhat hammered on taxes and fees. The original quoted price for a DONE4 via the online tool was 4,685 GBP. The "fare estimator" on the OW site indicates the price of a DONE4 ex-UK should be 4,139. So that's 546 GBP in taxes and fees I assume, or about $786. My issued ticket charged me $981.41 in taxes and fees. So I guess doing it in the US cost me about $200 extra. Not exactly great, but I was far more concerned about getting the base fare I was originally quoted.