Expensive polishes tend to last longer with less chips and cracks. This means your manicure lasts longer, which means you save time and polish. Cheap polish will start to crack the next day, especially if you use your hands for work.
Pricey nail polishes are thicker than cheaper polishes, so you may only need one coat. At the most, you'll need two coats versus three coats of cheap polish. Again, this saves time and money.
In my experience, pricey nail polishes have a much longer shelf life. This means you'll spend less money in polish because the expensive bottle will not separate as quickly.
Are pricey nail polishes worth the money? Yes, if you paint your nails every day and stick with a few basic colors.
I do buy inexpensive polishes in funky colors for my toenails where chips don't matter as much. They separate after a year, but by then, I'm ready for the next year's hot colors anyway.