Aparna,
By no means do you owe any commission. You posted on Craigslist to market the property yourself, not through a property manager. The fact that the tenant has an agent is of no bearing to you. The agent and the tenant should have had this conversation before they even looked at your house. If you do want to pay a referral commission out of the goodness of your heart, then a few hundred dollars is certainly ample. By no means should you give the agent one month's rent, that's way too high!

I have gotten commissions for putting a tenant and landlord together, but this was always agreed upon with the owner before I brought the tenant. I always ask for half of the first month's rent (paid to my broker), but I have settled for less. I have also done this for free many,many times. Renters may buy from me one day and homeowners may sell with me one day. I call this free advertising!

Normally the Seller/Landlord pays the commission. Ask the Agent what is there normal commission. The commission will vary from the cost of the monthly rental amount from $300-up to one months rent.

What's the agent for? To represent the renter in purchasing the house? In most cases the seller pays the agent a commission based on a percentage of the sales price. Usually, its 5-7%. If its for a renter, its not common, in my experience, to pay an agent for that. In Georgia, I don't think agents are legally allowed to take non-commission pay or "finders fees".