When we are talking about a "sequence of actions" it is correct to use the simple past. I would like to know if it is possible to use past perfect with the verb "spend" because first she spent 800 pounds before getting to Costa Rica- The key of the book says that the simple past is the only alternative but I don't agree.
Here is the text: Samantha... (spend)800 pounds of her own money getting to Costa Rica. First, she... (fly) to Miami. Then she... (catch) a plane to San José.
Thanks

Verb Tenses (http://leo.stcloudstate.edu/grammar/tenses.html)

It's been a long time since 9th grade but:

By the time she got to Costa Rica Samantha had spent 800 pounds of her own money.

Unless the textbook gives more indications or context, I would say you are right. Past perfect works "perfectly" in that situation.