I'm making $80 per day with a #4 ranking, how much to expect from a #1 ranking?

Hi guys,
So as the title stated, I am averaging around $60 - $80 per day from my Amazon affiliate niche site using Google as my traffic stream only.

I was wondering how much I'd expect to make from a #1 ranking if I invested money in SEO?

The keyword gets 18,000 monthly searches and the product value is $200+ so commissions are decent.

Would it be worth adding $1,000 for SEO link building to attempt to climb to #1 rankings or should I avoid the risk of losing my stable rankings which is generating me $80 per day?