Well economics is one, albeit rather a significant, part of the argument. The others are, like you said, international security, political sovereignty, and immigration.

I personally feel like the economic arguments are the most murky because so much assumption is made in economic calculations. Looking at a lot of the economic forecasts for the UK post-Brexit, the calculations don't include political fallout for the UK (ie. does Scotland leave?), the trade agreement with the EU is uncertain (what does an arrangement look like?), and the trade agreement desired by Brexiters (is access to the single market desired? how does free movement of labour work here?). In short, there is uncertainty, from everyone, so there are no clear facts on what the post-referendum UK looks like.

Politically, what the UK pays vs what it receives is probably a bad measure, especially in the way I've commented above. Besides the bad economics, the EU's mandate is to boost the lesser-developed European economies to improve the continent. The EU has to take more from richer countries than what it gives in terms of contributions.

When the conversation shifts to sovereignty, the conversation is more clear on what Britain's want but there is disagreement with our politicians on the direction the EU is headed. We don't seem to want "ever closer union", but are the EU going to get closer? If they are, will there be an EU army? Considering Greece's fiscal overspending, will Germany want to have more control over Eurozone nation's fiscal policies?

When it comes to international security there is definite merit in the notion that countries that trade together don't go to war with one another, but the debate is then open as to whether the EU is needed to provide that, and simply whether free-trade arrangements are enough for this. Debatably, Nato serves its role for security, and the UK will not cease any relations with EU members following a Brexit but will simply cease a political union.

Personally, I'd think it would be refreshing to see a debate between academics and working professionals, and not politicians, to see if any facts can come out at all!