Robert Rowe

I need horns pitched in "C". I am a multi-instrumentalist (keys, mandolin, guitar, upper brass, etc.), and I have trouble transposing "on the fly" (was never taught). When performing with my group, occasionally, I must pickup a line-or-so, when the vocalist "stumbles" a bit, and play the melody lines on trumpet. I have three fine "C" trumpets, but I prefer the tonal qualities of cornets. Schilke makes a "C" cornet, and there are the 100-yr old Bb-A-C changeable key cornets.
How much (approximate cost US $$) would a conversion job cost on a cornet ?? How do you determine the lengths of tubing (aside from the obvious slides) that necessitate shortening ?? In other words, isn't this a mathematical exercise ?? Or, just how is this type of job done ??

Robert Rowe