The current situation is like you giving me £1000 every month, and I'll send you £600 worth of food, clothing, other necessaries in return. Does that sound like a good deal to you? If so let me know and I'll give you my bank details. How are we not better off by not paying?
If you look at it purely in terms of net monetary value, yes you are right - taking into account the figures in that source, every person in the UK will be 26p per day better off if we were no longer an EU member. The question is, do you think that it's worth paying 26p per day to remain a member of the EU? For you, obviously not. Personally, I suspect it might be a price worth paying to maintain a position as an influencing member of the union, especially given that even if we do leave, a large amount of our trade will still need to be conducted with EU member states. For me, 26p per day to maintain that influence doesn't really strike me as a bad deal at all.