1 you must compare the exact brand, not just a generic compound.
ex OX by SAN at 2 or more different stores ( say DPS /1fast400 sells 4-5$ cheaper on DPS etc hence 15-20% cheaper on DPS; Vasopro sells at about 11$ on S&S while at about 7 at 1fast)
2 the reason varies between VOLUMES ( the biggest buyer gets the best prices from sources ), MARK-UP ( a vendor may be fine with 10% mark-up while for others 30% isn't enough, or 50% )
3 ADD-ONS. One store may offer more in-depth, personal service while another may just be reachable through some leased telemarketing phone center.
If you compare real, specific brands, i doubt you'll find such a gap as 80%/9$= almost 1/8.
The maximum i have found myself is 50%, ex one sells for 40, the other for 20.
If you go into powders, non specific or in-house brands, then you loose a fair comparison item.
If i were you, i'd shop name brands at the best prices.