be interested on后加什么？

Just out of interest, how much did it cost?

我只是好奇问问，这个花了你多少钱？

By that time I had lost interest in the idea.

那时我已经对此想法失去兴趣了。

Could I interest you in this model, Sir?

先生，请你瞧瞧这个型号好吗？