I've had a lot of experience with rain repellent on aircraft windshields. What I have learned over the years is that after rain repellant has been applied, it works great for the next flight, but becomes ineffective pretty quickly based on hours flown or, if the aircraft sits outside unused, the number of days that have passed since the repellant was applied.
So far as the motorcycle is concerned, I have found that inexpensive products such as Pledge furniture polish or Honda Spray Cleaner & Polish (mentioned above in post #10) work just as well as far more expensive products (including aircraft-spec products) for a day or two immediately after application, provided that the motorcycle is not subsequently washed with soap and water.
So, I just keep a small can of the Honda Spray Cleaner & Polish in the saddlebag, and if I am out riding and I think I am going to encounter rain, I clean the windshield using it before the rain starts to fall.
Bottom line: I don't think there is any product out there, at any price, that gives long-lasting repellent action.
More information, for those who are curious:
Aircraft Rain Protection
Michael