Justifying the cost of a support agreeme

My boss has decided not to renew our support agreement with our ERP vendor. Their level of service does not justify the cost .I have an offer for similar services from another company having competence in the ERP, for about half the cost. Our team has gained much expertise after implementing various modules of this ERP since 2 years. In fact we are supporting about 400 users internally. I fear that , by not planning for any external assistance, eventual problems with the ERP (so far stable) , may be such that our team might not be able to address. Moreover , the 2 resources are not full time support persons, as they are committed to other projects. What kind of metrics or other arguments can I put forward to my boss in order to justify this investment ?
Thanks for any help