If you need auto focus I think the cheapest option is the Olympus (full FT) 25 mm f2.8. But to be honest, I'd prefer to buy the 20mm f1.7 from panasonic which is only about 80 € more expensive than the 25mm FT.
There are many 24/28 mm legacy MF lenses available. With an Olympus OM adapter you can get f2.8 for about 10-35 € and f2.0 for about 50-100$ at eb.y.
AFAIK all brighter options are much more expensive.
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.
Cheap, fast and sharp 25mm? I think you get to choose two out of the three. I'm not sure there is something that is all three. Of course it depends what you mean by fast and what you call cheap (I know my idea of cheap is under $50 so that would severely limit your options). The Panasonic 20/1.7 is fast and you can pick the up for under $300 used so I'd say that that is as good an option as any.
The cheapest, fastest MF lens I have is the Olympus OM 24/2.8. It's sharp, not particularly fast and cost me around $100 (used) and I think hat was a bit under the going rate.
What about something like an old Series 1 Vivitar 24/2?
Another "cheap" lens, fast and very small: Computar 25mm f1.3. Being c-mount, a very small lens. Some vignetting, but you can pick this lens up for about $25 (plus an adapter, if you don't have one). Oh, and some careful grinding...
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