yah, sneaker streets is really big nono for asking your parents to help you buy stuff. basically, there is like the big big companies which will sell normal kicks for the "retail" price. while there is tons of unknown store names who sell both rare kicks and retail kicks. I suggest your mom to stay away from those ones as mostly they are re-sellers and jack up price like mad. *i remember some store who had green hemps for $9999 HK....*
Also, you have to shop around as most independent shops are acually owned by one "boss". So if they dont have ur size at a particular store, they will tell you to wait, pretend to go on the back room, and then quickly send someone out to their "boss" stores and grab you a pair. thats why i strongly advice you to not get your mom to shop at sneaker street if she does not know the right prices for the shoes you want.
There is one water shop near the sneaker street, I do not know what the name is, but is a few blocks away from the mong kok MTR station. They got numoures stores that sell rare kicks like SB's, dunk high exclusives, etc... but she has to look around and see which one is the cheapest. SHE MIGHT even be able to bargian a bit if they are nice. Last time, i went and bought a pair of SB's and they gave me 80HK off.