SEO budget

Want to know how much do the clients in niche gambling, health, e commerce will spend for buying backlinks per month on an average

Nobody can answer this question, it's too general and too hypothetical.

Most people just stablish a % based on the total budget for this purpose. But from my own experience talking with other colleagues, there are many schools on this topic. Some love content and spend like 80% content/onpage SEO topics and 20% for link building, and others are extremely focused on reputation and go the other way around 70/30%

By checking the linking profile of your competitors in a specific niche regularly you could infer approximately their budget on linkbuilding if you have experience with linkbuilding in general.

But as @MisterF has already stated no one should or would say you an exact budget because it's almost impossible to know this.