Can I start a sentence with "Just that", like:
It tastes good. (it's) just that it's too expensive.
Can I drop the "it's" at the start? If not, what's a good substitute that sounds casual and friendly? "Although" and "However" sound too formal in my opinion. Or are they?
It's fine. Some linguists refer to this kind of omission as a conversational deletion.
Conversational deletion is the tendency to omit portions of a sentence, usually the subject, when those portions refer to people, places, or things deemed obvious or implied in the conversation. —tag info for conversational deletion.
The conversational deletion is very common and perfectly idiomatic in speech. It's not likely to run afoul of any rule in informal situations.
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.
