I do that frequently, both buy and sell content. #seolife. The price depends on your niche, traffic and domain authority.

I've made a couple of deals, apart from the hand-wavy "find the highest price they're willing to pay you're also happy with".

I usually calculate the starting price using CPM (cost per mille ie. cost per 1000 views) and adjust up/down depending on eg. how likely the number of eyeballs is likely to go up.

This works for both your website and your newsletter.

A decent CPM for a website could be $3-5 up to maybe $10 CPM (remember that whatever the CPM, more eyeballs -> higher price). Newsletter is probably from $7-10 up to double digits as a CPM.