I thought GAT was the least expensive, but there are some things to consider.
How many days do you want to go, and if you are going before or after 5/28. Sometimes GAT is the cheapest when you look at certain scenarios. My spouse has access to TicketsAtWork through his company. In some scenarios they are less expensive, even with the service charges. GAT has a $10 service charge which would make less of a difference with a big purchase. Also consider AresTravel and Safeway/Vons(4 day not available here) and price it out.