Full Port Gate Valves
For valves manufactured to API 600 (generally cast valves, flanged and butt-weld, like is made by Crane Valve)
API 600, Sect. 184.108.40.206, Table 4, Minimum Port Diameters.
For valves manufactured to ANSI/API 602 (generally small forged valves, threaded, socket-weld and flanged, like is made by Vogt)
ANSI/API 602, Sect. 5.2, Table 1, Minimum Diameter of Equivalent Flow Passageway.
Generally, for forged valves per API 602, a standard port is a reduced port. As well as system block valves, these valves also include vents, drains and instrument connections. As far as system block valves are concerned, they are usually 2" and below for forged and for pipe sizes this small, pressure drop is usually not a problem. And yes, cost is a big factor; a forged full port valve body is the body for the next valve size up. The valve weight and cost can be double or triple.
For cast valves per API 600, the minimum port allowed is considered to be full port.
Note that 'full port' really means 'considered as full port for flow calculations'. Full port does not mean full size un-obstructed bore. Although the minimum bores are established in the standards noted above, the actual bore diameter will vary from manufacturer to manufacturer.
If you need a straight through bore of a certain diameter (for hot-tapping for example) you need to be specific about that in your purchase specifications and verify that on your approval and certified drawings. Don't trust catalog info, some major valve companies contract with valve manufacturers all over the world so the valve you get might not match the one shown in the catalog but it should meet all of the standards and specifications.