tm5.

There is no standard function you can use, you will have to write
your own (in a standard module):

Function MinimumValue(ParamArray Values())
   Dim MinVal as Variant, i as Integer
   If UBound(Values) >= 0 Then
       MinVal = Values(0)
       For i = 1 To UBound(Values)
           If Values(i) < MinValue Then MinValue = Values(i)
       Next
   Else
       MinValue = Null
   End If
   MinimumValue = MinValue
End Function

DMin works down rows, not across columns.

Your table should really look more like
Number
CostColumnNumber
Cost

Then the query is simple.  You can either use a custom VBA function or you
can use a normalizing union query to fix your data.
A union query cannot be built using the design view, but must be built using
the SQL window.

SELECT Number, Cost1 as Cost
FROM YourTable
UNION ALL
SELECT Number, Cost2 as Cost
FROM YourTable
UNION ALL
SELECT Number, Cost3 as Cost
FROM YourTable
UNION ALL
SELECT Number, Cost4 as Cost
FROM YourTable

Save that query and then you can use it as the source for a totals query.

Using a custom vba function - one posted below.  Copy it and paste it into a
VBA module and save the module - with a name other than fRowMin.

In your query, instead of DMin, use fRowMin

Field: LowestCost: fRowMin(Cost1,Cost2,Cost3,Cost4)

'================ Code follows =================
Public Function fRowMin(ParamArray Values()) As Variant
'John Spencer UMBC CHPDM
'Last Update: April 5, 2000
'Returns the minimum Number of a group of values passed to it.
'Sample call:  myMin = GetMinNumber("-21","TEST","2","3",4,5,6,"7",0)
'              returns -21
'Ignores values that cannot be treated as numbers.
'
'              Max of 29 arguments can be passed to a function in Access SQL
'               workaround is to nest fRowMin for groups of fields.

Dim i As Integer, vMin As Variant
Dim  tfFound As Boolean, dblCompare As Double

  vMin = 1E+308 'very large positive number
  For i = LBound(Values) To UBound(Values)

     If IsNumeric(Values(i)) Then
        dblCompare = CDbl(Values(i))
        If dblCompare < vMin Then
           vMin = dblCompare
           tfFound = True
        End If
     End If
  Next

  If tfFound Then
     fRowMin = vMin
  Else
     fRowMin = Null
  End If

End Function