-50% / +100%
The Boris figure is right to within -50% +100%.
The actual amount is only an issue to those who don't care about the principle at stake.
The EU cost could reasonable be as high as £660M per week: http://www.brugesgroup.com/blog/costs-and-liabilities-associated-with-the-european-union.
But even if there were some idiots who somehow thought the referendum was on an NHS budget increase of precisely £350M per week, there were plenty of others who actually read the question on the ballot paper before voting.
Even if the figure were not in dispute, as it was not on the ballot it would be part of the budget, debated in parliament (Gina Miller and the remainers would like that) and in fact could still be granted even now!
On the other hand, Ted Heaths lies were lies in principle, not of quantity, but none of those remainers who get so very excited about "lies" care about that, it was before their time!