In First Contact it is shown that four quantum torpedo's is enough to destroy a borg sphere. According to the DITL specs page it has a diameter of 600 meters. This would give a volume of about 113097335 m^3.

A borg cube would have a volume of 27983710656 m^3. Assuming an equal density it would take 990 quantum torpedo's to destroy a cube. However a transphasic torpedo has been shown to be able to destroy a borg cube with one shot.

Grahams Torpedo Yields Article lists quantum torpedo's as 50 isotons. 50*990=49500 so a transphasic torpedo would have a yield of fourty-nine thousand five hundred isotons, or approximately 122760 megatons.