Originally Posted by
LHR/MEL/Europe FF
You answered the question. 'Legitimate' was in the sense of actual cost (does it actually cost an extra $500 or whatever to power a jetbridge into place).
If it's airport fees it's just a mark up for convenience.
That's exactly what it is at most airports. And the cheapest of the cheapest systematically ask for bus gates, such as FR. TG, however, never appeared to market itself as budget no-frills airline.