A new "code of conduct" for consultants and others providing the Government with international development services has been published.
The move is designed to deal with the controversy over so-called "poverty barons" - individuals being paid large sums from the aid budget.
Two of the top earners are reported to command more than £200,000 a year each at a time when overseas spending is under pressure.
A Government commitment to maintaining funds to meet a target of spending 0.7% of national income on aid is controversial at a time of severe domestic spending cuts.
The code, which covers value for money, transparency and accountability, is voluntary but the Department for International Development (DfID) said the majority of its 12 biggest suppliers had signed up.
They will be expected to give written statements that they will seek "opportunities to reduce costs and waste" and publish information about where money is spent and with what results.
"If suppliers can't deliver better value for money, the department will work with other suppliers who do," DfID said - adding that more attention was being paid to keeping work in-house.
A spokeswoman said: "Taxpayers rightly expect DfID to deliver the very best value for their money - and sometimes this can only be achieved by bringing in expertise from outside government.
"But we are determined to get better value for money - and that is why we've introduced a new code of conduct for suppliers.
"The message to suppliers is loud and clear: you can only expect to work with us if you can prove you offer the best service at the right price."