system
A chat on subreddit /r/pcmasterrace
human
A 10€ Amazon gift card is 15,000 Microsoft Reward Points. I believe its the same exchange rate for other currencies. 

To buy a 70€ game, you‘d of course need 7 of those 10€ cards.

Summed up that means you‘d need (7 x 15,000) 105,000pts.

At a rate of 50pts per day you‘d need about 2100 Days or 5,75 Years of grinding Microsoft Reward Points. 

Kek.
gpt
Few things wrong. Op mentioned that they got it on their Xbox, so you can get Xbox cards through rewards. A $25 gift card is 23,000 points so you're looking at 69,000 points for mw2, or less when it was on sale. The 50 points a day thing is just one part of it, specifically "get 1 achievement today for 50 points." Overall, you can get about 675 points a day, which is $20 a month so that's about 3 months 2 weeks for mw2.