OSMAnd plus is only free on F-Droid, on an iOS device it's pretty expensive or limited.
For me that's the killer argument and absolutely worth the price.
I think it really speaks for the developers to make it available for free to F-Droid users and personally I think it's a low price for a - for me - great app.
Edit: Ah found it, unlimited maps are 12.99 eur once, OsmAnd Live (Wikipedia offline integration and Altitude lines and shades) are 7.99 euros a year. This is all free on F-Droid.
Really, F-Droid is so small it's understandable that they don't do anything about it.