Cantrips are always half a spell level in calculations.
Aha! To calculate the price, times the highest-level level spell by itself (.5*.5=.25), then times 200 gp (1/day) for a 50 gp cantrip Runestaff. Sadly, I can't find any Shadow spell cantrips.
addendum: 29-01-10, 11:25, -8 GMT, in any case, the funny thing is that it lowers its own price like that.
addendum 2: 29-01-10, 11:26, -8 GMT, for a level 1 spell 1/day, it's 200 gp. For level 2, 2*2*200=800 gp. level 3*3*200=1800. level 4*4*200=3200. level 5*5*200=5000. 6=7200. 7=9800. 8=12800. 9=16200.