It tastes good. Just that it's too expensive.
The meaning of the phrase "just that" in the sentence is understandable. However, the proper and complete phrase in the context should be "It's just that". You use this phrase when you are describing your reaction to something. For example:
I do love you. It's just that I am a bit surprised (McMillan).
So the sentence presented in question hould be as follows:
It tastes good. It's just that it's too expensive.
In other words, you can say:
It's not that it doesn't taste good, it's just that it's too expensive.