We were supposed to go to a Chinese buffet but when we phoned we found out that they raised the price to 15 (from 10 dollars), so we went to an Eastern Indian Buffet instead.
I was supposed to 〜 but... で〜するはずだったのだけど...した
I wanted to go to 〜 but... だと 〜に行きたかったのだけれど...に行った
We were gonna go to a Chinese buffet restaurant, but we called them to find out that they had raised the price from $10 to $15. So we went to an Indian buffet instead.
be supposed to を使うと、「〜するはずだった」とちょっと義務的な感じがします。be going to だと、単に「〜しようと思っていた、〜する予定だった」と、事実を述べているだけになります。