Beijing agents are usually much too greedy!

25 classes a week teaching high schoolers sounds far too many, irrespective of the salary offered for doing so! Aim for round about 16 a week, but for 16 a week I'd say 5,000 RMB a month would be reasonable if you're white and the company offers you an unshared rent-free apartment plus the services of a company-supplied teaching assistant. Just my two cents worth!