You cannot talk about cheap, fun cars without mentioning the Miata.
It's about as simple as a car can get. You get a four-cylinder engine up front, a stick for selecting gears in the middle (don't even think about the automatic), two wheels that propel the car in the back, two more that determine which direction the vehicle travels in the front, a pair of seats in the middle and a removable roof on top. The trunk was an afterthought. You can hardly fit a textbook back there.
The simplicity not only makes it cheap, but makes it simply amazing to drive. The steering is light and direct. The car is so balanced that it will just grip and grip. With the roof down you can experience the world around you, instead of just viewing it briefly through some glass. There isn't a ton of power, but the close ratios in the transmission give you a great excuse to rev the engine until your windshield is covered in oil and valves. You'd have to really try for that to happen, as these engines are incredibly tough.
In fact, they're so tough they have their own racing league in the SCCA. Many Miatas that race in the series are running virtually stock motors. Name a cheaper car that has its own serious racing league. What's that? You can't?
Yes, to shallow people this car is insanely girly and emasculates any man that drives it. The fact of the matter is this. There isn't a cheaper way to get a more critically acclaimed and respected car than the Miata. There aren't many cheaper ways to have a street legal car that you can go race every weekend.
Come to think of it... there aren't many cheaper convertibles either...