I've never printed wrapping paper. I suspect using my plotter for lots of full colour printing might be expensive!
Printing your own wrapping paper sounds like an expensive hobby to do the printing yourself, can't you use a printers?
The cost of ink and the time taken would make it a non runner, where as a printer will be able to make them at probably 100 times cheaper
Unless you are charging about £10-15 per sheet I don't see how you would make any money what with printer costs,design time, paper, ink, labour and selling costs