Regularly, a pair of Scourge cost 23/68, which after the -50 gas cost upgrade is 23/18.
With the Apex Predator prestige, combat units cost 25% more resources, which is an acceptable tradeoff with how powerful Zagara is. However, Scourge cost 29/86, which after the -50 gas cost upgrade is 29/36, double the gas cost of what you regularly pay.
The reason for this disproportion is that an additive modifier is applied after a multiplicative one. What I would suggest is changing the Simplified Genome upgrade to also have a 1.25x modifier, so that it reduces the Scourge gas cost by 62 (62.5 rounded). That would make them cost 29/24, which remains a consistent 25% higher resource cost of the prestige.
Realistically, no one pays the full 68 (or 86) gas price for a Scourge anyway. So in practice, you’re not paying 25% more gas for Scourge, you’re paying 100% more gas.