Your question as formulated is a little difficult to answer, because it requires a few data points that are a bit hard to gather. However, I may be able to shed some general light on this for you.
Background
First of all, it's important to understand that, much like the martyrs among Islamic extremists today, many human sacrifices in Aztec (and wider contemporary Mesoamerican culture) considered being a sacrifice a great honor, and went to be sacrificed willingly. Similar to Islamic extremists today, this had much to do with Aztec eschatology (particularly, guaranteeing a good place in the afterlife) -- which is outside the scope of this comment -- and also with societal pressures, but by no means was getting sacrificed a "bad" thing that the average man on the street was trying to do anything and everything in his power to avoid.
Having said that, people did get sacrificed who didn't want to be, especially if you consider captured victims from wars and prisoners. Aztec society was hierarchical, and it was possible to fall from a warrior (top of the chain) all the way down to a rank laborer or slave because of debts or crossing the wrong people. We don't know exactly which groups the unwilling sacrifices generally came from, but we do know there were some unwilling ones. We know this, because we have records from Aztecs who describe sacrifices as going poorly ("bad omens") because victims cried, screamed, evacuated their bowels, etc.
In addition, the Aztecs dominated the other Mexica surrounding, Nahua-speaking tribes around them and often took prisoners, slaves, and sacrifices from them. So, if you're including the larger Mexica surrounding populace as "average Aztec citizens" it was certainly possible that you could be forced into sacrifice against your will.
Numbers
As for numbers, that's a tricky proposition. We don't have a whole lot of reliable records prior to post-European conquest, as the Aztecs didn't really keep any, and the archaeological record is more supplementary than direct. Post-conquest records are likely to be exaggerated, as Europeans were aghast at the Mesoamerican human sacrificial practices (which, to be fair, were often extremely barbaric in the Aztecs' case, generally involving tearing out the victim's still-beating heart through an incision in his abdomen, and, at other times, the sacrifice of young children). However, according to contemporary European sources, about 84,000 prisoners were sacrificed over just four days during the reconsecration of the Great Pyramid of Tenochtitlan in 1487. Modern historians feel this number is exaggerated (it would be 14 people killed per minute), and the real number is somewhere closer to between 2,500 and 5,000 per day (still no mean feat).
Fernando de Alva Cortés Ixtlilxochitl, a native-Mexican historian of the Mexica Nahua peoples estimated that about 20% of all Mexica Nahua-speaking, non-Aztec children were sacrificed by the Aztecs every year. So, pretty high chance there (1 in 5), but this is an extremely high number and beggars belief (but would explain why the surrounding tribes were so willing to help Cortez and his invaders when they attacked the Aztecs). Victor Davis Hanson in his book Carnage and Culture, states that the sacrifices were likely closer to around 20,000 people per year.
Conclusion
All of this is mostly general info to help you understand that it's hard to answer your question definitively, because we just don't know for sure a ton about the Aztecs' sacrificial practices. Our sources are often biased -- and it has been argued that the Aztecs themselves intentionally inflated the numbers in order to intimidate their enemies and keep their own people in line. But I can say with a good deal of certainty that nobody was immune to being sacrificed -- indeed, people at the highest levels would sometimes volunteer for it for a variety of reasons.
EDIT: It occurs to me that I should emphasize an earlier point I touched on. You mention: "chances of getting sacrificed to the gods," but certain deities only accepted sacrifices of certain types of people, and only at certain times of year. For example, children were sacrificed to Tlaloc in February, whereas Xilonen's sacrificial victims were women in summer. So if you were in the right time of year (Aztecs' had an 18-month calendar) and were the right type of person, your chances of being a sacrifice were zero, at least for a while.
EDIT 2: I previously and incorrectly referred to the larger Nahua-speaking peoples in the Aztec sphere of influence as "Mexica" in this comment. Apologies.
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