But the BBC understands the UK's plans have come in for sustained criticism from EU officials.
The National Audit Office says the Treasury's estimate is "reasonable" but could easily go up or down.
Many in the EU believe that the UK may compromise on previously non-negotiable points.
A cross-party group of MPs aims to increase pressure for a Brexit customs union deal.
What we learned about the Leave result
Chris Morris tackles questions sent in about Brexit
Why is Brexit taking so long?
Some of the Brexit words are a bit repetitive...
What is Jeremy Corbyn and Labour's position?
What does Made in Britain mean for Brexit?
Why English courts are opening in the EU
What have EU leaders agreed so far on Brexit?
Focusing on the key lines from the prime minister
How much of the UK's trade is with the EU?