I was looking at doing something similar to this, car rental is about £50 for 2 days as well so worth thinking about as well.Finally booked accommodation today.
1 night in Wroclaw £50 (split between two of us)
2 nights in Leipzig (20 mins drive south) £290 (again split between two)
1 night back in Wroclaw £65 (between 2)
All sorted. Just need to buy the train tickets from Wroclaw>Leipzig return which are cheaper on the Polish train site than the German.