Embedding engagement
Costs: Value for money

It is likely that you will have to make viability decisions about every budgeted item in a project against the following elements:

Staff expertise: e.g. permanent staff who know the organisation and do not have to be paid for additionally may not have the direct skills needed. Consultants may have the exact skill set and be able to dedicate substantial time to the project but charge substantially more per day than the internal charge for a staff member.
Flexibility: e.g. you might borrow equipment from another office within your organisation but there might not be flexibility over availability. Hiring from an external source may give you more leeway.
Time available: e.g. you may be able to deliver the project in-house, but it may take more time as staff will still have other tasks they need to do. External consultants may deliver more quickly but their daily rates will be substantially more expensive than that of internal staff.
Community needs : such as inclusion issues versus costs eg of translating documents/ interpreters/ more or specialised facilitators.