When I was learning how to use だけ and しか, I was told that だけ and しか both means "only". But the latter requires the verb to be in the negative form.
I only like that person.
Only buy this.
But out of all the examples I've seen, だけ and しか are only used with nouns or verbs. I never seen it used with amounts.
A: This is expensive, isn't it?
B: It's not expensive at all! It's only 100 yen!
Is that correct? Also, can I use it wih other counter words like 一人 and 三つ?