I think many people give more when they’re reminded with calls, letters, visits, etc. Even advertising (with its associated costs) gets people to donate when they otherwise probably would not have. That’s the reality. In prewar Europe the Roshei Yeshivos and Rebbes also traveled within Europe and beyond (America, etc.) to raise funds for the Yeshiva. The travelling back then cost them money too.
Of course your point what percent of the raised funds go towards fundraising costs is a very relevant and pertinent question about whether it is reasonable. 10% most people might agree is reasonable. 50% most people probably agree is unreasonable and too much.
I recall a story from prewar Europe where one of the major Rosh Yeshivas went fundraising. He came to a gvir’s home to solicit tzedakah for the yeshiva. The gvir said how do I know my donation won’t go towards the costs of your travelling rather than towards paying for the Limud Torah. The Rosh Yeshiva gave him a good answer, though I forgot what it was (as well as which RY it was.)