so you wanted to spend $400 on 10 rasberri pi which doesnt include operating systems, cables or peripherals? remember you would need to buy sd cards and power cables too adds up. you would be better off having a budget of $500-600 and building a standard gaming computer. for about $600 you could run even new games on medium or high at playable framerates. for about $500-550 you could hit medium fairly solid. if all you wanted was low performance then you could drop your budget even further.
it would be cheaper and better this way.