What is at stake is our freedom as a nation. The economy is not the main issue, nor even is immigration for that matter. What matters is that we are a free people at heart and long to be free in our country again, to rule ourselves and not to be ruled by others. This is very simple, and the point cannot be over-emphasised.
That said, we can consider the effect on our economy of a Brexit, in a form which does not involve continued subjugation to the dictates of Brussels. Once we are out, or rather, as we are on our way out under Article 50, we should attempt to obtain a Free Trade Agreement with the EU, and it is reasonable to expect that we could reach an agreement by which we continued to trade in industrial goods without tariffs. Non-tariff barriers, especially in services, are liable to prove more of an obstacle.
It is obvious, when one thinks about it, that trade will be easier if everybody keeps exactly the same rules. The only way for everybody to keep the same rules is for there to be a single authority which sets and enforces them. If we desire to be free, and not be subject to such an authority, then there will be divergences in rules and regulations, and trade is likely to become a little harder. That is a price of freedom, and it is worth paying in my opinion.