I suggest you to buy the pack with 4 tablets (each 100 mg).... this is about 350 to 400 HK$.
If you choose one single tablet from a big bottle it is around 70 HK$, but this might be fake, at least I do not trust them.
100 mg is however a huge dose and just that it lasts longer a quater or a half of a tablet is OK for me,
propably it is as well better to reduce side effects.
There are even tablet cutter on the market (you can buy them just at the same place where you get Viagra).
(like this:
Tablet Cutter )