Working out in good athletic clothes does make a difference when you actually push yourself, but waddling on the treadmill at 3.0 for 20 minutes can be done in jeans. On my cardio days, I do wear dirt cheap compression underwear, a $2.00 cotton shirt and a pair of shorts I've had for about 5 years. Granted the shoes/socks I wear are priced a bit higher than standard gym apparel at ~$100 / $15 respectfully.