What is the best site to buy plane tickets online?

I want to book a flight for a one off trip, I googled my city to my destination and I saw listings from around $175 (spirit airlines) up to $500 (United and other airlines). What are the reasons for such a price gap in airline tickets? Is this accurate? What should I be expecting to pay from Chicago to Texas and... show more I want to book a flight for a one off trip, I googled my city to my destination and I saw listings from around $175 (spirit airlines) up to $500 (United and other airlines). What are the reasons for such a price gap in airline tickets? Is this accurate? What should I be expecting to pay from Chicago to Texas and what site do you recommend?
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