Negotiation

What are your thoughts on this situation?

For e.g:
Your cost price for a guest post site is $100, and you sell it for $130, including article charges. Yet, when the client demands $50 and you deny it, he says he bought it for $10 from another seller?

Have you ever been in this situation? What is your approach if this is the case?

"Thank you for considering buying my service, but you're not part of my target audience. I/we serve clients from $xyz budget and above. Have a good day!"

In this way, you don't "close the door" for potential future deals, but also keep your service's value (and, anyway: you don't really need a client who would pay this small amount of money).

I'm not some kind of businessman but:
1. If you are seeing such problems frequently your target audience is wrong.
2. Your margins are very slim. Unless this service has high velocity there is no room for improvement and growth.
3. Always try different marketing channels and never stick to just one.