It does sound plausible to me. My last DONE5 was over USD1000 in "taxes" and I did not have any UK stopovers (thus no GBP80 per stopover).
A recently ticketed 20 segment DONE4 was about USD500 in "taxes". Again, with no UK stopovers. The big difference is due to (a) ticketing on AA for the recent one, and (b) the previous one include several high tax destinations.