You cannot make $100 in a single day from a forum, you cannot even earn $100 in one month. There are not many forums available for paid posting. Even if you work on multiple forums and work for the entire day, you might not even make $5. When I started on paid to post sites, I was earning around $60 per month, sometimes I even managed to earn $90 in a month. However, most of the high paying forums have stopped paying or has cut pay rate.