Glaze materials really are cheap no matter what they cost if you think about it.Especially feldspars.
At least that what I have found.
Mark is correct. Just remember that any feldspar you use as a glaze component is a complex mix that might work as well or better with a combination of other (cheaper) materials of approx the same chemical composition. the ONLY way you can find out is by testing and tweaking. Cornish stone does not have any wonderful, inherent properties that would be impossible to get otherwise. Software calculations MAY get you closer, but you probably will have to fine tune to get just the results you want. This means understanding how to move the gaze in the direction you want by manipulating the components. There is plenty of info out there and as a newbie it is greatly to your advantage to learn at lest the basics of glaze formulation.
Start out with Min's formulation from Digifire and go for it.