How to decide maintenance cost/terms for freelance work? [closed]

Question

I am a JavaScript programmer and am planning to take Freelance projects related to JavaScript effects for the web. Recently a potential client approached me to create a JavaScript effect for his website. The effect was quite complex and it was not generic. That is, every time the design of the website would change the JavaScript code would have to be modified.

Due to this nature of the code, I told the client that I will not be able to provide maintenance for this project. I will be happy to modify the code in the future but I will be doing the cost estimation each time there is a modification. The client was not happy with this.

I thought of calculating the probability of the modifications the client may ask per year and multiply the initial estimate by that so that the maintenance becomes free. For example, if the cost of the initial project is $100 and I assume that the client will ask me to modify the code 4 times a year I would charge him $ (100 + 4*100). But I think this would be way too much and the client would deny to pay such a high amount.

I have just started freelancing and the cost estimation part related to maintenance confuses me a lot. What is a good way to estimate maintenance cost. Also suggestions on maintenance terms like what should be the scope would be useful.