Should I use ‘was’ or ‘is’ in the following paragraph?
“My sister saw a blue dress at the mall yesterday. She loved it very much. But the dress was too expensive for her. It was one hundred dollars. She didn’t have enough money for it.”
You should use past tense in your given context.
In your text, you are recounting an experience which occurred in the past, so you should use the past tense.
On the other hand, if you are having a casual conversation at the mall with your friend, you can say "This dress is very expensive, it is over 100 dollars". In this context, present tense would be the preferred tense.
I upvoted Max, but let me add ...
You use past tense here because you are describing an event that happened in the past. If you said, "The dress IS one hundred dollars", that would mean that it is $100 now. But you probably don't know that. Maybe at the time you are telling the story, the dress is on sale and is only $50. Maybe the price has gone up since then and it's now $120. And in any case, telling us what the price of the dress is now doesn't tell us what it was at the time she considered buying it. The price today is irrelevant to the story.
If you were talking about something that will never change -- or at least that is not expected to change any time soon -- you should use "is". Like suppose the story was, "Our friend Sally asked my sister to visit her in Los Angeles. But my sister lives in Tampa and Los Angeles is 2,547. That was much too far for her to travel." Note I said the distance "is". The distance from Tampa to Los Angeles is not going to change any time in the foreseeable future. So the distance at the time the story happened, the distance today, and the distance 20 years from now are all the same. We say "is". But I said "WAS much too far". At that time it was too far for her to travel. Maybe at some time in the future she'll be willing to travel greater distances, maybe not. So that's "was". It was true at that time. It may or may not be true at the time I tell the story.