Please see the demo file covering number of scenarios showing how to Rank all values or aggregated values; or rank on more than one field.
Some of the formulas used:
Rank function Syntax: RANK(value, field, isAscending, includeNulls, dataSubset)
 
Ranking Spend=
RANK([Spend]) 
# simple Rank comparing all values in the Spend field  
 
Rank Spend per Week=
RANK([Spend], [Spend], false, false, SUBSET([Week])) 
#Ranking Spend values inside each Week  
 
SubRank Clicks =   
RANK([Clicks], [Clicks], false, false,subset([Rank Spend per Week] )) 
# secondary rank field, based on the Clicks value. Meaningful when multiple rows share the same [Rank spend per Week] value 
 
Spend per Keyword=
SUBSET_SUM([Spend], SUBSET([Keyword])) 
# total Spend for each Keyword 
 
[One]=1 
# basis for running total indexing 
 
Running total per Keyword:
RUNNINGTOTAL([One], SUBSET([Keyword]))  
 
Rank Totals per Keyword=  
IF([Running total per Keyw]>1, null,
RANK([Spend per Keyword], [Spend per Keyword], false, true, SUBSET([Running total per Keyw],1,"="))) 
# ignoring the values greater than 1, rank compares only the subset sum values where [Running total per Keyw] is 1, ensuring that only one value per category is compared.