tipota

what can i do to add more horsepower to my sentra? its an old car with 100k miles on it, so i dont want to invest a lot of money that id rather save for a new car. all mods ive searched for for sentras are expensive compared to other cars. my friend got a cold air intake on his accord for $30, and so did my friend on his civic, but the cheapest ive found for my sentra was $400. any suggestions?