- fermented【形】発酵した (definition from Eijiro on the Web)
× the cost is cheaper in Hong Kong.The vegetarian food in Hong Kong is generally more delicious, too.
○ the [food is/restaurants are] cheaper in Hong Kong.
○ the cost is lower in Hong Kong.
"Cheaper" means "lower in price". "Cost" is a synonym for "price". That means that the incorrect sentence above means:
- The price is lower in price in Hong Kong. ← strange!
"Cost" or "price" is a number, so you should use "lower" or "higher". Use "cheap" for products.
If you happen to know a good recipe or tips for cooking char sui at home, please let me know, especially if you know how to cook the vegetarian version. The cost of making it at home is certainly lower than flying to Hong Kong to eat it again. If you know a way to get cheaper tickets to Hong Kong, though, I'd like to hear that, too!