Tks for your advice, anyone knows where can I get good price buying Raynox 250?
Hold your horses! Your 18-200mm cannot support the Raynox 250 as it's diameter is 72mm where Raynox 250 attachment is only for 52mm to 67mm diameter only.
If you only have the 18-200mm lens and you want an affordable macro setup, I suggest that you get a 50mm f1.8 and attach Raynox 250 to it. If you have more budget, you can get either the Canon EF 100mm f2.8 USM macro lens or the newer Canon EF 100mm f2.8L HIS USM macro lens or Tamron 90mm f2.8 macro lens.