I used to buy OPI nail polish all the time, cause everyone said it was the best. I was wanting some new neon and black colors, so I bought some cheap brand called NYC (New York Colors). Its in Target and I'm sure lots of other places. They're like 94 cents.
To my astonishment, they last really well. If you apply two coats and a top coat of shiny, it will last quite a while. They last longer than the six dollar and up brands. And they have great trendy colors.