Rebecca,

Different industries have different "customary markups". Many industries today do what is called keystoning. This means whatever you pay for something you double it to get the selling price.

Don't confuse markup with gross profit. A 100% markup is only 50% gross profit.

Utimately, you'll have to determine what it'll take you to keep the doors open and return a net profit and figure this into your pricing model.

In my own company we try to average a 40% gross profit margin. Some things we may only make 10% on and others 60% on, but on a quarterly or annual basis, our weighted margins average 40%. This covers the overhead and returns a fair net profit.

A good place to start will be by determining your break-even point.
Hope this helps.