I really want to, but I got hundreds of things to do. / I'm tied up all day that day.
我真的很想去，但是我有好多好多事情要做。／我一整天都很忙。
I am sorry but I have to turn you down.
不好意思，我不得不拒绝你了。
I am sorry. I am not interested.
不好意思，没兴趣。

Thanks. Just looking. / OK, I'll think about it. / I can't afford it. / That's a little over my budget.
谢谢，我只是随便看看。／嗯好的，我再考虑下。／我买不起。／这有点超过我的预算了。
I do love it, but I'll need to talk it over with my wife/girlfriend.
我很喜欢，但是我得和我妻子／女朋友再商量一下。
There's too much workload here. I can't handle it.
工作太多了，我应付不来。
There is nothing to talk about. / No comment.
没什么好说的。／不想说，无可奉告。