The Uk contributions - that 350 million gross / 250 million net - come to about 0.7% of UK GDP.
Where the money goes:
https://europa.eu/european-union/topics/budget_enn_
Admin, including pensions:
“About 6% of the budget.
This includes administrative costs for all the institutions (mainly the European Commission, Parliament and EU Council), including the translators and interpreters needed to make information available in all of the EU’s official languages.
As part of a reform package, the largest institution, the Commission, is cutting staff by 1% every year, while also increasing working hours.
Ultimately, staff will be cut by 5%.”
The total EU headcount is around 30,000 to serve over 500 million people,
which compares favourably with 450,000 UK civil servants for 65 million.
Thee are estimates of the increased headcount that will be needed post-Brexit, but it will probably be more than the 4,000 or so that represent our share of that 30,000.
The cost of creating from scratch and reproducing all the EU agencies, inspecting facilities, infrastructure etc will take a big chunk out of that "250 million" savings
And that's before we consider the cost of the trade reduction that Brexit will cause, at least for a few years post-Brexit
the reduced tax take for public services and UK regional investment, especially if corporation tax must be slashed to try to keep businesses in the UK