The cheapest way to go to Windows of the World from Regal Hong Kong Hotel?

You can take Island Line from Causeway Bay Station to Wan Chai Station. There are several coaches that can take you to SZ Bay. The fare is 4.4 HKD per person. The schedules are from 7:10 to 19:40 with the frequency of 30 mins. It costs 40 HKD per person. After arriving, take public bus no.90 to 1st stop of WOW. Then walk southwest around 130 yards (120 meters) to find the scenic area.

In addition, you need to have China visa and then you are allowed to enter SZ. It's suggested to apply for it in advance in your country.