Dixon pays Victoria $2000 (i.e., $200 more than his contract price with Mason) to get the 1000 green hoops. But he makes $2500 on the contract with Madonna and avoids liability for breach of contract. He comes out at least $300 ahead.
Dixon's purchase of the green hoops was reasonable; he cut his losses and, since he purchased immediately after the breach, he did not unreasonably delay the purchase.
Suppose instead that Mason had paid $3000 for the hoops from Victoria's Wholesale Hoops. Doing so would have increased his losses and would not have been reasonable.