I have been looking around for cheap flights going transpacific, but have only found Zipair so far. Are there any other Transpacific ULCCs that serve the mainland US? Thanks!
You can figure this out easily using Kiwi. Simply start a search for direct flights between the West coast of North America and Asia and you’ll have the list of cheapest flights. These won’t necessarily be “ultra low cost” airlines but it doesn’t matter - you’re after the lowest price, not a specific airline type.
The cheapest flight right now is with Zipair from LA to Japan for $664 round trip.
That's a somewhat broad question. There are a lot transpacific routes but almost all of them originate (or terminate) in North America.
The busiest routes are probably between Japan and US/Canada west coast. That means there is lots of competition which tend to keep prices a bit lower. This being said, both countries are considered "rich" which means the airline will try to charge more.
One of the factors here is distance: Transpacific means at least 5000 miles. While airline prices rarely depend on the actual operating cost of the flight, the operating cost sets a minimum price.
Currently the the going rate for US<->Japan in November is about $900, which seems reasonable. With Zipair you can indeed find prices in the $700 range, but by the time you add a checked bag and a meal, you are at the same price level as the mainline carriers.
Flights from the US to other destinations in SE Asia are currently a bit more expensive ($1000-$1100) but that typically involves also larger distances.
One way to lower the price is to start from a low-cost country. MNL<->LAX can be had in the $800 range, but LAX<->MNL is at least $1000.
The concept of a Low Cost Carrier (LCC) is somewhat confusing. Typically it means "we are stand alone, don't sell connections and don't partner with anyone". It also means "you have to pay extra for food, seat, bag carry on, toilet usage, etc.".
It does NOT mean "We are cheaper". In my experiences LCCs are hit or miss. Sometimes they are indeed the cheapest option but often they are not. The assumption that "LCCs are cheaper because they are LCCs" is in my experience mostly wrong. In fact: one of my current main routes is served by a mainline carrier and a LCC. More often than not, they are price matched down to the dollar. However the LCC charges extra for carry on, and the mainline doesn't. Easy choice in this case.