The Swag Pack, of which there are 250 available, costs $60. It includes three different shirts (including an officially branded QuakeCon tee), a QuakeCon-branded Logitech G400 Mouse, and a $10 voucher for use on Bethesda's site.
The Swag Pack is available in conjunction with a "Bring Your Own Computer" Select-a-Seat, which nets you a reserved spot in the special BYOC section for $90, with only 500 available. You can also get a BYOC seat along with just a "QuakeCon done Quick" pass, which will grant you express lane access to all of the show's panels and events. That package with a single shirt is available for $50, and 250 are available in total.
Finally, if it's just a BYOC reservation and shirt you want, you'll pay $30 for one of 1,250 packages available.
All package options and prices are listed in full over on the official website. QuakeCon 2013 will be held at the Hilton Anatole in Dallas, Texas, from August 1 until August 4.