Apparently converting XLSX to CSV and keeping formulae intact is not something you can Google and find out how to do it? This is how you do it.

xlsx2csv.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'csv'
require 'roo'

excel = Roo::Excelx.new('filename.xlsx')

CSV do |csv| # (stdout)

  excel.sheets.each do |s|
    sheet = excel.sheet(s)

    1.upto(excel.last_row) do |y|
      row = []

      1.upto(excel.last_column) do |x|
        if sheet.formula?(y, x)
          row << '=' + sheet.formula(y, x)
        else
          row << sheet.cell(y, x)
        end
      end

      csv << row
    end

    csv << ['----------']
  end

end