The answer to your question would be that the conjunctive adverb that would complete the sentence is the following one: However. Therefore, the sentence would be as follows: Franny wanted to go to Spain; however, she could not afford the exorbitant plane ticket.
"However" is used to introduce a statement that contrasts with or seems to contradict something that has been previously said. In this case, it contradicts with Franny's desire to go to Spain.
