When tea is getting made, the tea leaves need to be steeped in hot water, which means they need to sit in hot water. "Too steep" can mean too high. The joke is referring to the price of tea being too steep, or too high. The joke is playing on the word "steep" which is a word used to describe how tea is made, and also refers to how expensive the tea is.
I was going to buy some loose tea , but the price was too steep .