Your only real options for the PC right now are ActLabs and HK EMS. Act Labs will set you back around $200 for a dual gun setup,and the HK EMS TopGun will set you back around $130 depending on where you buy it and shipping.
There are some interesting options that are supposed to be available sometime soon, but no word on when or what the price will be. A rough estimate of the price is it will cost more than the ActLabs guns for a dual setup.
Both available guns have their pros and cons. If you get a Topgun that works well and manage to get it setup, you'll probably be happy with it. The cons are that they can be very difficult to get working, the distance you need to stand back is greater than with a normal arcade gun, and they are not as durable as an arcade gun.
ActLabs... Pros- They seem to be easier to setup and are adequately accurate. No sensor bars or ugly wires hanging off your cab bezel. Cons- Costs more, only works with CRT displays. Also requires a special adapter for use with a TV or PC. Will not work with with LCD or Plasma.