Edit: I realised after posting that this is a 10 month old topic that was ressed because winniebaer edited the origional post.
A reaper at 850 cost and 6,800 HP is 'heavy'. Spy does 8000 EMP damage at 280 cost. The spy makes 3x cost against a reaper.
A sumo at 2200 cost has 12,500 HP. It requires two spies. Spies make ~4x cost.
A dante at 3500 cost has 11,000 HP. It requires two spies. Spies make 6.25x cost.
A Bantha at 10500 cost has 36,000 HP. It requires 5 (4.5) spies. Spies make 7.5x cost.
A Detriment is 24000 cost. Over 10x the cost of a sumo, so you would expect it to take 20 spies. It only takes 11, lets call it 12 to be safe. Spies make ~7x cost.
The spy is a highly effective anti-heavy unit. For-cost comparisons are all that count. Raiders will beat riots, and air beats AA, if you have enough. What matters is a for-cost comparison.