Got it for around 10 € at Berlin airport. When it comes to nail polishes OPI is a true pro, lasts for ages.
TIP: to prolong any nail varnish clean your nails with an alcohol swab before painting them. It will remove any traces of grease that prevents nail polish from sticking properly to your nails. Alcohol might dry your nails out though, so skip this step if your nails are brittle or prone to chipping and cracking.