FWIW I have found it more costly - I think this almost has to be true, as $X given to charity is $X I cannot put towards savings, mortgages, etc. - but, owed to fortunate circumstances, not very burdensome to deal with. I expect others will have better insight to offer.
Given your worries, an alternative to the GWWC pledge which might be worth contemplating is the one at The Life You Can Save. Their recommended proportion varies by income (i.e. a higher % with larger incomes), and is typically smaller than GWWC across most income bands (on their calculator, you only give 10% at ~$500 000 USD, and <2% up to ~$100 000).
Another suggestion I would make is it might be worth waiting for a while longer than "Once I have a job and I'm financially secure" before making a decision like this. It sounds like some of your uncertainties may become clearer with time (e.g. once you enter your career you may get a clearer sense of what your earning trajectory is going to look like, developments in your personal circumstances may steer you towards or away from buying a house). Further, 'experimenting' with giving different proportions may also give useful information.
How long to wait figuring things out doesn't have an easy answer: most decisions can be improved by waiting to gather more information, but most also shouldn't be 'put off' indefinitely. That said, commonsense advice would be to give oneself plenty of time when weighing up whether to make important lifelong commitments. Personally speaking, I'm glad I joined GWWC (when I was still a student), and I think doing so was the right decision, but - although I didn't rush in a whim - I think a wiser version of me would have taken greater care than I in fact did.