Cost for Filing of GST

It would start from 500 and could go upto 10000 per month or even ahead of that.

Depends on your earning capability, CA firm professional service and quality of services.

Yes, It would start from 500 and could go upto 10,000 per month or even more than that.
It's all depends on your earning capability, turnover, nature of transaction, industry in which you operates, CA firm professional service and quality of services which you are getting.
Some Agents are filing return at very low cost also but you don't get quality of service from them which results in adverse consequences in future

Yes ,agreeing with both
it's varies from Rs. 500-10000pm and many other factor also as your area location, Your firm credibilty(As more quality, More Money)
Soo it's all depend on you

It completely depends upon the Firm from whom you are taking services. It also varies from state to state. But usually it starts from Rs. 500 and there is no such maximum limit.