Netflix is designing original programming to fill holes so 25% is probably low. The three month assumption is also very low as the subscription is cheap and people don't think lets cancel right after a show they liked. Further they now own these shows so they gain long term benefit from having them.

So, a failure is probably worth ~10m and a success can be worth 20+x that. Remember eventually they end up with a back catalog that's large enough to be self sustaining.