How much to charge for another website to advertise on my own?

Mine is ranked no4 and theirs is as no5.

They contacted me to advertise their website on mine through a button, and said they will pay a flat fee of $600pm for a start. I've got about 25k unique visitors pm.

I'm a bit anxious as this might switch the rankings if I add theirs on, and I honestly this is really important to me, given this is basically my hobby website.