Most writs consume less than 1000g worth of materials for each writ voucher they return. But some consume 4000g+ materials per voucher and are probably not worth doing.
Shopping guild stores for Master Writs? Which is cheaper, 700,000g for a 72-voucher writ, or 400,000g for a 45-voucher writ? Sure, you could do division in your head or with a calculator. Or let the computer tell you (it's the 400,000g/45-voucher writ, just barely).
This add-on shows adds text to each Master Writ's tooltip, showing the total material cost, purchase price (if any), and the cost per writ voucher reward.
It also has an option to dump a complete material list to chat.