As a Leica user of some four decades, I would never place an inexpensive, i.e. cheap, filter in front of my Leica lenses because of a number of factors: (1) they may not be plane parallel; (2) the quality of the filter's glass will be less than that of the lens you're protecting; and (3) you could easily run into the problems of reflections - internal or otherwise. In short, why go to the point of purchasing excellent equipment and then degrade your images.
Personally, I would opt for a B+W pro filter with anti-reflection coating or perhaps a Leitz filter if they make one for your camera.
Even if they are a little expensive for your budget, save your money and then purchase it when you are able.
Best regards,
Bill