Might be better to talk about this in terms of conversion cost rather than probability. To turn a gamete into a person you need another gamete, $X worth of miscellaneous raw materials (including, but certainly not limited to, food), and a healthy female of childbearing age. She's effectively removed from the workforce for a predictable period of time, reducing her probable lifetime earning potential by $Y, and has some chance of various medical complications, which can be mitigated by modern treatments costing $Z but even then works out to some number of QALYs in reduced life expectancy. Finally, there's some chance of the process failing and producing an undersized corpse, or a living creature which does not adequately fulfill the definition of "person."
In short, a gamete isn't a person for the same reason a work order and a handful of plastic pellets aren't a street-legal automobile.
A few notes about the site mechanics
A few notes about the community
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A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site.
(Note from orthonormal: MBlume and other contributors wrote the original version of this welcome message, and I've stolen heavily from it.)