I usually buy my glasses to correct my far sight at the northern part of Temple street in YMT or Aplui Street in SSP for about 80hkd. The quality and style is not too bad. Reading glasses are much cheaper off course.
If your eyes aren't the same strength you can probably buy two pairs and exchange one of the glasses (heat up frame in hot water, push glass out and in).
Sometimes I go to the new wave of cheap optical shops. Prices there start at hkd 160. They measure your eyes, but are very bad at that.