If I prioritize my money in a material thing (Cars, house, toys etc), that's where my affection goes. If I put God first when it comes to my money and my time, it says, "God, you're first in my life."
If you show me how you spend your time and how you spend your money, I'll show you what's important in your life. No matter what you say is important -- show me your schedule and your checkbook stubs and I'll tell you what's really important to you.
Paul is saying in these verses that although he is very thankful for the Phillipians and their gift to him, he is not preaching the gospel for the money and he would do it even if they didn’t give. And the Phillippians, by their gifts, are saying that they are investing in the gospel which is something bigger than themselves.