I assume a fair few do what I do, and buy a season ticket just for away games. I’ll probably make 3-6 home games a year from London depending on kick off times and how well we’re doing etc.
Whilst the £400 or so for a season ticket is expensive, it is affordable in the context of away day costs. My £400 season ticket will probably cost the same as the combined cost of a fan travelling down to the Watford, QPR and Millwall away games.
Also getting to games from London can be easier in a lot of circumstances than from Sunderland. If we ever got to the Prem it would be even more advantageous, given half the league is south of Watford.