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As an answer to your immediate question, if rent was advertised at $1,200 per month after you left, that would not be conclusive proof that the landlord was forcing you to pay for the cost of the pest control for the whole building, however, it is evidence that could be used in conjunction with other evidence, if there is any, to make this case. The difficulty here is that as a month to month tenant, the landlord is legally permitted to raise your rent like this. And if you leave, he is then also legally permitted to market the unit for whatever he rent seeks, and then also to reduce that amount if he has trouble finding a tenant.
I am sure this is not the answer you seek, but I am obligated to provide you with the proper legal analysis, so I help this is helpful. Please provide a rating so I receive credit for promptly responding to you. After rating, feel free to ask follow up questions. Thanks!