Keep in mind that the primary customer for swagelok values and actuators is the industrial process control world and most often used in petrochemical and other industrial plants. Such values have to work 24/7/365 in exposed environment and meet the many NEC electrical safety area classifications. So while expensive from a home hobbyist point of view they are competitively priced for the market they serve.
I too would have you re-look at using large R/C hobby servos. They are avalible in high torque versions and are probably the most cost effective method short of possibly looking for used industrial equipment on E-bay and such. I would think trying to home build a actuator would be very time consuming with a high risk of failure.
Lefty