I don't think that Sage50 has the option to mark up based on the cost price. Pricing is set manually, but then you can use the customer price lists to adjust for each client. You can use the Price List blanket update feature to update all prices in a list by a %-age, but again, this is not based on the cost price.

This would be a great feature in some industries, but a nightmare for others.

Under Setup | Settings | Inventory & Services | Price List | Update Price Lists, one of the options is 'Cost'.  This part of the software can only be accessed in 'single user mode'.  

Sage 50 is designed to keep track of inventory items that you regularly buy and sell, more easily than materials that you pick up for individual 'jobs'.

The best way to keep track in Sage 50, is to be diligent about adding materials purchased to the appropriate Sales Order / Work Order, and Project (if you use that).

If you're looking for the part of the software that works something like what QuickBooks does with attaching random non-inventory purchases to a customer 'job', then showing you what to 'bill' on that 'job', it isn't there to find.