Though it depends on various factors like the size of RAM, it also depends on the beta build version.
The idle usage of RAM on a Asus T100 machine for Windows 10 was less compared to Windows 8.1 (Mar, 2015). So with less resources it can run the processes, while the remaining can be used for gaming and other memory intensive programs.
Though, it’s very early to compare the 10 Technical Preview memory usage, but 17% is better than 20%. Related : How to Change Startup Settings in Windows 10/8.1/7ġ0 and 8.1 used about 17% of the memory, which comes to around 700 MB.