Pricey brands may burn up your wallet. Most of their products are priced between 80-120USD. You could get big discounts during their clear-out or holiday sale, but new arrivials are normally costly.
Cheap leggings are often seen from 12 to 30USD at regular prices, on Amazon or Ebay. But be careful with those choices and read through their reviews.
Somewhere in the middle are brands with price between $40 and $80 online and offer decent quality sports clothing if you want to save up yet still have a bigger chance of getting quality stuff.