how much money one would have to spend to reach 3 Diamonds
I'm an engineer. I like back-of-the-envelop calculations.
Worst case: You buy credits in 50 credit blocks. You buy only 3K cards at list price with your level discount. $4,822.84
Really cheap case: You buy credits in 500 credit blocks. You buy the cheapest cards you can and only when you can get a 25% discount on top of your level discount. This means you only buy 720p cards until you reach platinum. Then you buy all 259 VG classic (480p) cards and finish off with 720p cards: $1,047.31
Nominal Case: You buy credits in 500 credit blocks. You buy 1080p cards until you hit diamond and then you switch to 3K. You buy on sales but not exclusively averaging a 10% additional discount: $2,125.81