What Tim Challies teaches on sacrificial or grace giving was exemplified by the Philippians in 2 Corinthians eight and nine. Challies writes in an article: Money Matters: How much do I give?

If you are giving an amount that really doesn’t even impact you—you make $10,000 a year and are giving $100 or you make $50,000 and are giving $250 or you are making $50,000,000 and are giving $20,000–your giving is not truly sacrificial. Giving is meant to be felt. If you aren’t feeling your giving, if you aren’t having to put other plans on hold because of your giving, you are probably not giving enough. There is a special kind of thrill that comes at the end of a year when you look at what you have given that year and see sacrifice. You can see that you could have had a new computer or a new kitchen or a new car, yet you’ve chosen to serve and honor the Lord. That is a God-honoring sacrifice. God loves that. God blesses that. And there is the second principle: At a minimum, give enough that it makes a difference to your financial position.