We want it to feel cheap when we buy but not when we give.

Psychology behind $0.99 for paid apps, but $1.00 for donation apps?
For Example:

People have (immense) faith in God. In place of immense, what would be the proper word to convey that 'the faith is not immense, but it is that which can't be given up, due to the fear of being alone. Even having less faith in God, we can't give up' ?
