Go through your fact finding notes from Prime Mover, participant, end-user and vendor interviews, looking for every non-routine resource that must be included in the project budget. Include any information you've captured about when the resource will be needed.
Separate the list into 'All or Nothing' and 'Variable' categories (AoN means that you've either got the resource or you don't while Variable means that there are levels or degrees of the resource that could be used in the project).
For the items on the Variable list that represent a significant cost, estimate a Mini-Max 2x2, where you list the minimum and maximum amount/form of the resource (and the related costs) in the top two blocks and likely impact on project return of these two extremes in the bottom two blocks. Be as precise (read 'credible') as you can be with these estimates, but don't let your need to make an educated guess keep you from Mini-Maxing.
Present the preliminary project budget to the Prime for discussion, focusing any negotiation (Prime's frequently want more for less), on the Variable items and their impact on project outcomes.