Huh? Dude, your numbers didn't work. They just didn't. The probabilities of finding an inhabitable planet in Andromeda and in the uncharted areas of the Milky Way are the same. Plus the chance of no Reapers in Andromeda.
If you forget the mass relay system (which may or may not exist at that point) then yes, the probabilities are the same. They're nonexistent.
Even if there were a billion garden worlds, that's a **** ton of systems you need to comb with conventional FTL drive to find them.
With the FTL that exists during ME3 (~50xspeed of light) the travel time from Sol to Proxima Centauri would take about 1 month, give or take. And that's to the nearest star.
So yeah, let's comb 50 years through a thousand star systems at random and we may or may not find anything.
That'll make a fun game...