When you offer something to God, it tends to be sacrifice. That's right. That's good. Because offering something to God isn't like offering something to a beggar. You give a beggar a few bucks and are in a position or benefaction and power over him. What's more, you probably don't miss what you give because you don't think about giving sacrificially in those moments.
But God, He's a different story. When I give to Him (money, time, abilities, and so on), I'm not playing the role of benefactor nor exercising some power over Him. He's not once looked at me and said, "O! Thank you for tithing! I wasn't sure how we were going to make it without your check! Whew! Praise Me!" Never happened. Never will.
Giving to God is sacrificial by nature because in our giving (again, of time, money, abilities, etc.) we're giving to someone far superior to us to reflect His worth to us. And when the lady dropped the two small copper coins (21.1-4), she was reflecting God's worth to her. That's why Jesus praised her. That's why He set her out as an example.
So what is God worth to you and how will you reflect it in the sacrifice you give?
But that's just me thinking thoughts...