We didn't add to dc's travel fund at all. She was responsible for every penny that she earned (in a minimum wage zero hours job) and for almost every penny that she spent. dc had a target amount she wanted to save and when she approached that, she bought a ticket and off she went.
We were also clear that in managing her money while she was away, she needed to ALWAYS have enough to get home (although of course we would have paid the earth to get her back if anything went wrong).
Of course she lived rent free at home during the time she was saving, and I was generous with treats - but all her day-to-day expenses such as bus travel / socialising / phone remained her responsibility.
imo, (and relevant to my dd - it is different for other people's dc) her gap phase was as much about learning to make good decisions about money (money which she had earned) as it was about travel and experience.
For my dd (and other people's dcs will have different life lessons to learn) this approach meant that when she went to university, she was clear about what she could afford and what she couldn't; she had learned how to manage the interaction if friends around her seemed to have more disposable income; she was clear about her priorities for spending on clubs and activities and on socialising. She was fortunate because she always knew that we were there as a safety net (both while she was travelling, and while she was at university), but we were a safety net, not a funding source.
Tbh, I'd rather save the money to help with deposit for rent, / house / whatever than make her gap time a little more luxurious and comfortable.