class Order < ActiveResource::Base
  self.site = ''
end

o = Order.new(:price => 6.47)
o.to_xml
# => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<order>\n  <price type=\"float\">6.47</price>\n</order>\n"

o = Order.new(:price => BigDecimal.new('6.47'))
o.to_xml
#=> "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<order>\n  <price type=\"decimal\">6.47</price>\n</order>\n"

# with the same precision you use
o = Order.new(:price => BigDecimal.new('6.47', 4))
o.to_xml
#=> "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<order>\n  <price type=\"decimal\">6.47</price>\n</order>\n"