Onion prices dropped more than a fifth on Thursday afternoon after the government allowed duty-free imports and restricted overseas sales of the bulb,three traders said. Average onion prices at the country's largest wholesale onion trading hub in Lasalgaon,in Maharashtra,dropped by nearly 22 per cent to Rs 1,800 per 100 kg on Thursday. India on Wednesday allowed duty free imports of onion after banning its exports earlier this week.