You can adjust your prices in several ways. For example, by setting surcharges, using tiered pricing, or creating a custom attribute in which you calculate your new prices.
What do you use best in what situation?
Would you like to raise or lower all your prices? Then use surcharges.
Would you like to offer quantity discounts for a selection of products? Use tiered prices.
Want some more flexibility in setting your prices? Use custom attributes. This article is specifically about creating new prices using custom attributes.