Add a percentage to the price for certain gig extra's

[mjakkerman]

I run a gig for writing texts (of all kind). Most orders are quite simple, writing texts for web pages or something, but sometimes a customer requires a researched article. Sometimes these posts are quite short (let’s say 300 words) and sometimes customers request longer researched articles (2000 words). I would like to put a ‘gig extra’ for researched articles, but $5 is too much for a 2000 word article, and maybe a bit too little for a $40 dollar article. Ideally, I would like to be able to charge a percentage for researched articles.