Hi Rochy,
I can't comment about the respective costs, but about the only thing that Metformin and gliclazide have in common are that they are both diabetes medications! They do totally different things and are not interchangeable.
Metformin is a biguanide medicine that works by reducing insulin resistance and suppressing the production of unwanted glucose by the liver. Gliclazide is a suphonylurea medicine and it works by forcing the pancreas to produce insulin. In other words metformin doesn't create insulin but gliclazide does. Gliclazide doesn't help the insulin to work properly but metformin does. Gliclazide is supposed to be prescribed with metformin, not instead of it!
Instructions on prescribing of gliclazide are that it should be started at 40-80mg per day and gradually increased at fortnightly intervals to the maximum dose of 160mg per day.
I hope that it is not your "DN" who is doing the prescribing because she appears to have very little understanding of the various medicines and what they are used for. I am very concerned both at what you have been told and what you have been prescribed, and can only suggest that you go over her head and demand to see your GP.