The Samsung Galaxy S4 might be a new phone, but there are already some great deals to be had online. Amazon previously lowered the price to $168, but now it's down to just $129.99 for new lines, and $139.99 for upgrades on existing ones.
These deals will, of course, require you to sign up for 2 years of service. Just make absolutely sure this is the device you want to use the upgrade on – AT&T recently changed the upgrade eligibility term from 20 months to 24 months.