Hello Kirby, like Iain already explained, I'm afraid the Mega Pack discount offer was for a limited time only, as a gesture to those who had already bought the GC's and wanted the Mega Pack as well.
So like you already said, basically you have the choice of going for the Mega Pack, or purchase the remaining 8 GC's. I think the Mega Pack is cheaper than the 8 other GC's together, so my recommendation would be to go for the Mega Pack.
And if you end up with unused GC's like me, you can use those to ensnare another hapless victim into this addictive game

. After I got the Mega Pack I had installed my old GC's on one of my other PC's, which also has AK on it.
One of my friends is an old-time Panzer General fan who already had the base game and had little interest in the GC's. But out of curiosity he had to have a look, so I let him play it at my place. He now owns a complete set as well... But as he explains, all the money he has spent on the game would not be enough for a single friday night out in town, so he still considers it guite good value for the money.