I'm not sure in which of the following two paragraphs you are interested. Pick the one that applies.

If the site takes a percentage (say 12%) of the total:
$freelancers_fee = $total * (1 - $site_cut)
which is equivalen to
$total = $freelancers_fee / (1 - $site_cut)
Example: Site charges $150, leaving $132 for the freelancer.
Example: Freelancer wants $100, meaning the price has to be $113.64

If the site takes a percentage (say 12%) of the freelancers's fee:
$total = $freelancers_fee * (1 + $site_cut)
which is equivalen to
$artist_fee = $total / (1 + $site_cut)
Example: Freelancer charges $100, bringing the total to $112
Example: Freelancer wants the site to charge $150, leaving $133.93 for the freelancer.