I have spent 1 year on my first project - X3 map by Scorp. It is totally free, no limitations, just a free wish to donate. There are 20.000 of happy user of the program and less than 10 (!) donations from them. Tons of thankful replies but no donations.
My second project - VTM, i have spent 1,5 years for working on it. Many happy users. I must admit that VTM approach for donations was better but still not the better solution. I decided to change this.
And i believe 10$ is not the sum to discuss.