By Jonathan Spicer and David Henry

NEW YORK (Reuters) - The minister taking the first tricky steps to re-negotiating Britain's trade deals in a post-Brexit world said on Tuesday he is taking a methodical approach and first needs to ramp up his staff to around 300, from 40 now, over the next few months.

Mark Price, the UK's Minister of State for Trade and Investment, said in an interview in New York that he was meeting with U.S. policymakers and investors to start imagining what "the shape of" free trade agreements may be once Britain ultimately leaves the European Union.

Britons voted last month to leave the block and Price, who had campaigned against such a move, stressed it would take time since formal negotiations with a swath of countries from the United States to China to Turkey cannot begin before more is known about Britain's ultimate relationship with the EU.

For a start, his department needs to bulk up with negotiators and others.

"We've just started the process now, we're building our organizational structure, recruiting the people to come in. You can't do that overnight," he told Reuters, adding: "We think 300 is where we need to get to."

Asked about the cost of hiring Britain's new negotiators, Price, a former managing director of supermarket chain Waitrose, said he had no estimate but added, "trained negotiators are going to not be inexpensive."

Nearly half of UK exports go to the EU but beyond that, Price said the United States and China are priorities for trade negotiations.

Over the next 12 months Price's team would start working through the options for a U.S. deal, and the Transatlantic Trade and Investment Partnership (T-TIP), which the United States is now negotiating with the EU, is "a great piece of work to build upon," he said. "I suspect we'd push for even greater liberalism within it," Price added.

Any such deal would come after the November U.S. election, in which Republican presidential candidate Donald Trump has campaigned on an anti-free trade platform - echoing the sentiments of the Britons who wanted to leave the EU.

Price noted that Trump has business interests in Britain. "I don't think that at the moment I'm thinking about one candidate or another and how they might feel. Both candidates have been warm toward the UK," he said.

"At the end of the day it comes down to cold, hard economic logic... (and) on that basis you end up getting to a sensible place" on trade, he added.

(Reporting by Jonathan Spicer and David Henry; Editing by Bernard Orr)