I think they ripped you off on the math. 24*60=1440 ghash. You should get your money back or get the full 25 units.
Each MR is basically 8 Singles in one case. It's really 480GH/s, not 500GH/s. 3x8=24, not 25. Why would they give him an extra unit just cuz he wants his hashrate in 2 dozen smaller cases rather than 3 big ones?