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.