It needs to be crystal clear to the mother exactly what the nature of the Placement is - is it a supportive placement or is it an Assessment placement? Given the vulnerability and care needs of some younger mothers it is unfair to place them under intense pressure within a mother-and-baby foster-home placement where the mum will feel under extreme scrutiny and pressure to the extent of not being able to parent her baby 'naturally' - she will always be seeking to please / satisfy the foster-carer. It will also have a direct impact on the nature of the mother-and child v foster-carers relationship.
Within a Residential Assessment Unit there will be other mothers and babies, more opportunity to relax and behave more naturally and even more opportunity to make mistakes safely and be able to receive staff help, advice and feedback in a less threatening way than on a one-to-one basis within a foster-home where the mum will have no-one else to turn to and will feel unable to do anything other than comply and be submissive.
Costs are not the issue - it is the quality of the assessment and the skills, knowledge and expertise of the assessors. A Unit may well have access also to a number of different professionals rather than end up relying on what may the subjective view of just the foster-carer.