Choreboy is about to add...

...ridge vents because they are the most cost-effective method.

Moreso than a gable vent with the same free area?

That's what I've read. If you have rige vents and soffit vents, you
have several feet of height for convection, and your air movement is
spread out under the roof.

My question was whether it's less expensive to install (say) 2 2'x2' gable
vents than a 2"x48' ridge vent, on an existing house, with soffit vents
in either case. Also, it seems to me that 2 2'x2' soffit vents may be more
cost-effective than continuous soffit vents, with equivalent performance.

If the soffit vents are in the center, air will spread out under the roof.
If not, convection currents through the large free "vent areas" inside
the attic will tend to equalize the air temperature.

You might keep your attic 100 F on an 80 F day with a 1000 cfm fan,
moving about 1000x20 = 20K Btu/h out of the attic, OR use soffit
and gable vents with area A and an 8' height difference, where 20K
= 16.6Asqrt(8)20^1.5, ie A = 4.8 ft^2, with no fan.

With A = 8 ft^2 (the 2 2'x2' vents), 20K = 16.6x8sqrt(8)dT^1.5, so
dT = 14 F, and the air coming out of the gable vents would be about
80+14 = 94 F. With soffit vents near the gables, the air in the middle
of the attic might be dT warmer than the air near the gables, where
10K = 16.6x48ft^2sqrt(8)dT^1.5, so dT = 2.7 F, for a 4/12 roof.

My 100' greenhouses ventilate well with roll-up sides and gable vents.

Nick