item: {
   id: 1,
   variants: [
    {
     id: 1,
     prices: [
      {
       needle: 100
      }
      {
       needle: 200
      }
     ]
    }
   ]
 }

item.variants.Select(v => v.needle.ToString()))

item.variants.Where(x => x.prices != null)
               .SelectMany(v =>
                 v.prices.Select(p =>
                   p.needle.ToString()
                 ).Distinct()
               )