Yes how much can you do with that?
I think ordinary people often have it hard to get a good "proportional impression" in regards to goverment finances. Sometimes a few million is made to appear like a lot of money, and sometimes a few billion is thrown around like it's spare cash. I must admit i have my scrutiny on the very first point that Brexiteers put out a figure that was on a weekly basis as imho "it's made to sound more like it really is" rather than the norm to put it on a yearly basis.
So some figures:
UK GDP 2828 billion dollar*
Uk goverment revenue 810 billion pounds*
*2018 figures
SO, if we take a figure of 150 milion pounds and multiply it by 50 we arrive at a figure of roughly 7.5 billion pounds. Thats about 0.2% of GDP and almost 1% of the goverment budget. Meaning that if Britain as a result of Brexit looses out on something between 0.2 and 1% of growth over say the next decade outside as a modification of it's normal growth rate, aka if the British economy slows down "a bit" that figure might turn into a loss soon? To be fair on the economy the size of Britain that 150m/week figure is rather marginal in size, it's like saying "we will get some spare cash back".