The first step is to determine what support arrangement you have with Microsoft? Who is authorized to create Microsoft support incidents? Does your organization have assigned Microsoft technical support managers? Microsoft can help with product questions and issues that arise from software updates. In most cases, they will not be familiar with your specific implementation.
Next, do you have a help desk or do you need to hire an outside company to provide help desk services? Do you need coverage during business hours or 24 hours x 7 days a week? Will your users call your help desk to report their initial issue?
How will you handle enhancements to your solution? Do you intend to publish new versions on a periodic basis? At what interval (weekly, monthly, quarterly or other)? If so, how large a team is required for these iterations? Will the team that did your implementation be available?
For how long a period do you want to budget? You can expect the level of enhancements to drop after the first year unless new business requirements are introduced.