how much more should I charge for swiss meringue buttercream than american buttercream?

You need to work out how much both of them cost you to make both ingredients and time and charge accordingly.

AThank you. I've always made American buttercream but have trying my hand at Swiss and Italian meringue buttercream, so I was unsure if I should charge the same as American

AYour cost is what should drive the price you charge.

Using numbers I grab out of the air...

Each batch will ice cakes with 100 servings

American Buttercream costs you = $10/batch (or $0.10/serving) Swiss/Italian Meringue Buttercream costs you = $15/batch (or $0.15/serving)

So right off the bat you could add the difference of $0.05/serving to all Swiss/Italian iced cakes. Now, if those icings take longer to make (meaning more of your time is used) than American Buttercream, don't forget to tack on extra $$ for that too.

Thank you for your thread, pricing has been asked many times previous on the forums, please start here:  

http://cakecentral.com/t/764720/policies-for-posting-the-cake-decorating-business-forum

Then I encourage you to check the stickies at the top of the business forum for pricing matrixes as well as using the search function to read the thousands of other threads dedicated to pricing.