If your sole reason for moving is to reduce your rent, moving in closer to Philadelphia from Downingtown isn't going to make that goal easier, $1100 is already the lower end for rent on a decent 1-bed in metro Philadelphia. Deals nearer to train stations can be found but you'll either have to hustle or you can't be too particular - e.g. pick less than nicer neighborhood, take shabby/dated unit, garden unit, duplex or triplex with thin walls/floors.
If you want cheap, look beyond the Main Line (Ardmore, Narberth) to Delaware County. Hunt along the two train lines that head to Media and Newark DE, in places like Lansdowne, Secane, Ridley Park, etc. You're going to be more likely to find something as low as $800 (but these are not necessarily walkable towns.)
Other notes:
Manayunk isn't a suburb, it's a neighborhood within city limits. Parking is also a real issue in many areas of Manayunk if you plan to keep one car, let alone two.
Note with the Phila wage tax - it's currently 3.47% for non-residents and 3.9% for for residents - less than a half a percent difference. I don't know what you make but even someone with a six-figure salary still spends considerably more on a monthly regional rail pass with Septa than on the difference in wage tax. (Not sure how you're calculating the 2% now and 2% additional figures?)
That said, if you realize the greater commute savings by moving downtown close enough to walk to work, you'll be hard pressed to reduce your rent. While Philly has the 3rd-5th largest residential downtown (after Manhattan and SF it's debatable for 3rd/4th/5th against Boston and Chicago), it's not cheap. Center City under $1100 is close to being a non-starter unless you accept a small studio or tiny unrenovated trinity and that would be a very rare find in an affluent neighborhood like Society Hill.