From the campaign:
You've just unlocked the FREE UPGRADE #5 on the Heavy Troopers for the Kruganova Shock Troops and the Iron Empire. Yeahh!
The next one is at $420,000, the Box of 3 Kurganova Snipers.
Upcoming unlocks will be in the next Update! Stay tuned! And yes, the $425,000 will be another FREEBIE!