My mother bought my sister a present that is too cheap and I'm going to let me mom know.
What is your reasoning for telling your mother that it is too cheap?
My mom is always talking about "good" expensive things
Even though your mother may want to know what is going on, you should not put down others because they cannot afford a gift that is more expensive. It is not the cost of the present but the thought behind it.
You don't get how I feel. You need to get presents for people that they like and I feel this present is cheap.
Your mom might be struggling financially and putting her down for not purchasing a more expensive gift would be wrong and embarrass her. The gift was for your sister and it isn't your place to put insert yourself and potentially cause drama. Your mom was considerate to give a gift and you should leave it at that.