I don't think it is possible to do a meaningful odds breakdown.
There are more forest restricted sites of nature., more waste restricted sites of fire and death. Earth and to some extent air sites tend to be more common in mountains.
Wastelands are generally your friend. Mountains, forest and swamps a bit so, while farms have decreased chance of sites.
I would guess that about 75% of all sites are lvl 0-2, 24% 3 and 1% lvl4. But of the lvl3 sites many are rare and thus less likely to be in tha game. Perhaps 80%-90% are sites findable by a lvl 2 mage. On the other hand, it is as Omikron says, the rarest ones you really want to find
Some sites are unique and there can only be one on each map. Some of these are more common, meaning it is not unlikely that it exists on the map, while other uniques are very rare as well. Sites linked to terrains are more common in the sense that if there was forests all over the place they would be common, but since they are restricted they will not be that common. Since most maps have few swamps you could say that the swamp sites are rarer (many of them are water or nature).
There might be a list of sites somewhere in the modding threads. I don't think it presents rarity, but might display path requirements.