A good wig can cost $40 or $4,000 depending on what you need. "Good" does not always mean human hair, and expensive does not always mean comfortable.
Budget synthetic
Beauty supply synthetic lace fronts can be surprisingly wearable in the $40 to $100 range, but they usually need trimming, de-shining, or hairline work.
Everyday realistic wigs
Better synthetic, lace-front, and monofilament wigs often run from about $150 to $600. These are popular for daily wear because they hold style and require less work.
Human hair wigs
Human hair wigs commonly start in the hundreds and can move into the thousands. You pay for hair quality, cap construction, length, density, knot work, and customization.
Bottom line
A good wig is the one that fits your use case. Spend more for comfort, realism, and longevity; spend less if you want color play, occasional wear, or a beginner experiment.