Are you just trying to get to the end?
Are you trying to buy every car available?
Are you trying to get 100% on every series?
I'm now halfway through Spirit of Ferrari in the Legend series and I've started thinking about my end game. If I keep only doing enough to get to the next stage, then what have I accomplished? By the time I get to the end of the game, I will not have completed all of it nor will I own all the cars.
Many people start skipping cars in Legend as they cost so much GC. If I skip the 20% discount now, I will just have to pay more later when I realize that I didn't accomplish much by getting to the end. I now regret skipping cars that were around 100 GC in earlier series.
For me, I will focus on the series that must be completed to continue, not the sub-series that have cars I don't need for now. I will buy the cheaper GC cars and wait for the others to go on sale. 30% off 300 GC and less cars isn't that big of a deal compared to 20% off. On those 600 GC and above cars, it's a bigger deal.
What's your end game and how will you accomplish it?