The copper tacks which are cheaper than the brass tacks on your canoe are not canoe tacks. Dan Miller has 2 lbs of copper canoe tacks advertised in the want ads for $58/ lb plus shipping. Regular copper tacks are too thick to turn back into the ribs and clinch properly.
Use copper if you foresee any use in salt water; brass tacks will corrode (de-zincify) in salt water, becoming weaker and staining the surrounding rib wood with white blooms.