I Want to Flatten my Top Sharp 'Canine' Teeth. How Much Should Should This Cost Per Tooth?

What you are asking to have done is called enamelplasty.  The cost should be minimal to have done.  I would have the dentist make the changes in small increments since if they take off too much you cannot put it back again. 

While it's a relatively simple treatment to cosmetically contour your 2 upper canines (in the hands of an experienced cosmetic dentist), that should not cost very much (up to a few hundred dollars), the dentist must make sure NOT to compromise your occlusion by doing so. The canines "protect" other teeth as they travel across their opposing teeth when you chew and move your jaw around. Make sure the dentist takes that into account.