As someone who works out regularly, it's good to have cute outfits for the motivation but at the same time be price conscious because you'll be wanting to wash these clothes a lot, sweat isn't a friendly thing.
I'm quite rough on my workout clothes seeing as I'm workout obsessed - working out at least 5 days a week for 2-3 hours.
I find lululemon too overpriced for what you get. Definitely invest in a few workout bras, kind of like how we should have a good foundation for our personal wardrobe (aka spend more on bras) this still is true for working out. I wear Shock Absorber bras because I can adjust the tightness since I need a 30D. I hate when my girls bounce, and I've been known to double up, wear a workout bra and then a nike top with built in support. I'd stay away from any cotton except for underwear.
I like to match, so I stick with solid black bottoms (black also doesn't show sweat as much as lighter colors I've noticed. I go to nordstrom rack and shop the sale workout rack at regular nordies. I usually do a stretch test, you want it to bounce back quickly because these are definitely supporting garments and should be worn tightly but not too tightly. Nothing is worse than watching a girl trying to hike her pants up as she tries to walk/run.
Also invest in good quality socks and shoes.
My priorities for workout clothes are: 1) Bras, 2) Shoes 3) Socks 4) Clothes