Hello Srinivasan
The Dual homed method 2 involves two separate ISP routers from the SAME ISP. This would most likely increase the cost of the solution compared to method 1 because of the fact that you require your ISP to provide you with two pieces of equipment on their end.
Method 3 would most likely be even more expensive, because you are purchasing connectivity to two DIFFERENT ISPs, so you can’t take advantage of any discounts the single ISP may give you for multiple links.
What I’m describing is the most probable case concerning cost. Ultimately it depends on the billing policy of each ISP.
I hope this has been helpful!
Laz