NO bivvy bag will leave you condensation free 100% of the time, its physically impossible in some conditions
Humidity, temperature, dew point etc all play a part. In an ideal world it'll be warm inside and cold outside, and very low humidity.
If your sleeping bag is keeping all the warmth inside and against you, then there won't be enough heat between your sleeping bag and bivvy bag, so the bivvy bag itself is cold (as its 'touching' the outside air), so the water condenses against it (think condensation against the windows inside your car or house). The warmer the bivvy bag is
A cheapish bivvy bag clearly won't help, you want to give it as much help as possible to remain condensation free.