Sun Gai Gweilo was complaining recently about the price (and availability) of English language books in Hong Kong. And, yes, I am sure I will be criticized for writing this, but English is an official language of Hong Kong, so you might hope that books would be readily available at a reasonable price. And they're not.
In Hong Kong, it's standard practice for bookshops to convert US Dollar prices to HK Dollars 10:1 (rather than than the actual exchange rate of 7.8), and a similar mark-up applies to British prices - though the fluctuating exchange rate makes this more difficult to track.
The thing is that even if they used a fair exchange rate, you'd still be paying more (for most titles) than anyone would actually pay in the UK - discounts of 30-40% are widely available in the UK, particularly on new titles and from online booksellers.
The answer ought to be online, but once again we seem to be poorly served in Hong Kong. You can order from Amazon, but delivery charges are quite steep. However, their service is excellent, and books arrive a few days after ordering. Paddyfield only charge a modest amount for delivery, and use the real exchange rate, but delivery on many titles takes up to 4 weeks, and they don't offer the range of titles or the discounts that you can get from Amazon.
The explanation is simple enough - lack of demand and lack of competition. If you go to other major cities in Asia, such as KL, Bangkok and Singapore, you will find a better choice of English books at more competitive prices than in Hong Kong.