It is still possible to get curation rewards with little SP,although the chances are small. The chances would still be higher with let´s say say 70 SP,compared to 20. And if you want to get to 200,why not break the investment down to several payments. If you believe in the platform in the long haul, this would make economic sense,as you are increasing your earnings by interests gained by holding SP,and by higher curation rewards.