Your question's answer depends on whether you are looking for a user plane, i.e. something you intend to use, or a collectible plane that you want to show off or resell for a higher price. The latter is very complex, but the former is straightforward. The easiest answer is to purchase a premium, new plane and call it a day.
If you purchase an old plane, you will have to expect to put some work into it such as derusting and cleaning. A number 4, a smoother, requires a bit more work than other planes. The sole should be as flat as possible to ensure very thin shavings. Don't expect it to be flat when you buy one. It can take anywhere from an hour or more to flatten the sole using cheap and easily obtained materials. There are tons of online resources that explain the process.
Under no circumstances should you buy one that has any size cracks, large chips or deep pitting. If you see any, kindly refuse. Such defects can compromise its function or its structural integrity. I would only buy such planes with the intent of salvaging it's parts that are in good condition.
The price of the plane depends on a variety of factors, including type, model and overall condition. It is hard to say without looking at it. There are some #4 that are very rare, fetching hundreds of dollars, but it requires a good eye and experience. $15 is a reasonable price for a number 4, even for a less desirable newer post war type.