Tithing is tax so that the priests will have something to eat, so that churches will be repaired and built. It's not law, you can give when able. If it's painful to tithe then maybe, its like cable tv subscription you don't use, or the channels are not reaching you.
A prosperous church means a lot of people tithe because they feel the church gives them something valuable. If the church is poor, that means, its not giving anything relevant to people.
People are drawn to give. The 10% rule is just some priests who want 10% share of the communities wealth to benefit the church's institutions. Nothing wrong with it, priests are people too.
It is exactly like a payment for services rendered,and indeed it is a service. That's what its called, right?