Yvo de Boer, head of the U.N. Climate Change Secretariat, also told the BBC World Service in an interview that rich countries needed to pledge deep cuts in greenhouse gas emissions by 2020 and the poor had to slow the rise in their emissions.
But cash was needed to kick-start a deal.
If we can get in Copenhagen something like 10 billion euros or dollars on the table that will allow developing countries to begin preparing national plans to limit their emissions and adapt to climate change, then that would be a good beginning, he said.
But even more importantly, Copenhagen has to agree an architecture, a burden-sharing formula, that will allow us to share out the costs of climate action among countries as the needs increase over time, he added.
Costs of fighting climate change in the longer term could be up to $200 billion a year, according to U.N. projections.
Developing nations say the rich have to show willingness to give cash to launch a new U.N pact to succeed the Kyoto Protocol beyond 2012.
Many developing nations are likely to be hardest hit by climate change such as more droughts, disease, floods, heat waves and rising sea levels.
Environmental group Greenpeace said that far higher figures of about $140 billion annually should be on the table when leaders of the Group of 20 discuss climate finance at a meeting in the U.S. city of Pittsburgh in September.
De Boer is absolutely right to highlight that this finance question must be resolved to break the deadlock in the international climate talks but $10 billion could only be regarded as a down payment, Greenpeace campaigner Joss Garman said.
De Boer said that rich nations were finding it harder to come up with cash because of the recession. It's become more difficult to raise financial resources, he said.
He also said developed countries should be guided in planning emissions cuts by what he has often called a good beacon of reductions of 25 to 40 percent below 1990 levels by 2020.
In a 2007 report, a U.N. panel of scientists said cuts of 25-40 percent were needed to avert the worst of global warming. So far, promises by developed nations amount to cuts of only about 10 to 14 percent below 1990 levels by 2020.
And de Boer said that developing nations had to sign up to slow the rise of their emissions, mainly from burning fossil fuels in factories, power plants and vehicles, as part of a Copenhagen deal.
If on that piece of paper, China, India, Brazil and other major developing countries have offered national actions that will significantly take their emissions below business as usual ... that for me will be a success, he said.