The Panasonic 20mm f/1.7 is your best option. You haven't specified what cheap is but the Panasonic gives you fast, it is within your focal lenght range, is quite sharp and generally a pretty good performer for a reasonable price. That's why I bought one.
Now my other lens in that range is the Voigtlander 25mm f/0.95. Now that is FAST, probably one of the sharpest lenses in MFT and a great peformer. It isn't cheap and if you are not use to using manual focus lenses, maybe not the right lens for you. I think it is exceptional value as there is nothing like it in current production lenses that will give you that sort of speed for anwhere near the price. Maybe it is cheap after all.