Paying to Access Custom Fields Per Post

This sounds like custom development work, I doubt there is a pre-built solution. At its most basic, I’d say you need to a membership solution. There are a several out there, some free, some premium.

Then in your front-end you can wrap the display of all the custom fields with current_user_can() using a capability that only that level has. For custom user roles you can probably just get away with:

if( current_user_can('gold_member') ) {
 echo 'whoa check this out';
} else {
  echo 'sorry, no need to be a gold member to see this';
}

EDIT

or to make it post specific:

if( current_user_can('read_post_14') ) {
 echo 'whoa check this out';
} else {
  echo 'sorry, no need to be a gold member to see this';
}

You’d have to create a new role for each post though…. possibly on the save_post hook. But I’m not sure how you’d assign the role to user when he/she pays for a specific post.