I'd roll my eyes at $75 for an entire story of that length and not even bother approaching someone that obnoxiously demanding and how little they value the writers work in the scheme of things. Since that's about the going price per article for small publications. My BF has done freelance for articles before, he was cheap at $50 a story when he was starting out, that includes going to an event on a press pass and writing about it for the publication.
If you want more information about a specific ad, you have to ask. Most places will negotiate with a writer, since they are not in full control, this is a business transaction. You are a vendor offering a service, you set your prices, the customer is more than likely going to lowball you in that kind of fashion, they can dream of paying whatever they want.
Don't rely on a broad net of "this is how it works for me" though because in every single freelance situation it's going to be different. Each client is going to expect something different and pay differently, you can either say "this is my fee, I don't have room to negotiate" and work like that or say "This is my normal fee, I will offer you the work at this reduced cost to benefit us both", it's all about how you want to run your business.
Do not let anyone walk over you or treat you like you're less powerful in a situation because they're paying you. You are a business in this sense, you are selling something of value that they are desiring, that's why they're placing ads in the first place.
It's like every business. People will call and ask me "How much for this?" "It's X amount." "Oh that's too much." "Well that's how much it costs, so you can pay it or go to another location, good luck finding something for nothing!" I had someone say they wanted to get rich and hire me to raise their children, then were shocked when the going yearly salary for a live in nanny is NOT just a few hundred dollars a month in pocket change