I'll risk the wrath of those who think we're too late....
As EU members, we could expect to enjoy
1. Unfettered access to the largest trade block in the world
2. Free trade deals with countries around the world, including Japan, Canada and South Korea
3. Just-in-time manufacturing, the kind that supports millions of jobs in our automotive, aerospace and other sectors
4. Wide-open border between Northern Ireland and Ireland, with no customs or other checks between NI and the rest of the UK
5. Support for the Good Friday Agreement & active promotion of the Irish peace process, including PEACE funding (over EUR 1.5 billion between 1995 and 2020)
6. The freedom for UK citizens to travel, work, study and retire anywhere in the EU without needing visas
7. Scientific and academic collaboration
8. Shared space exploration
9. Participation in the Galileo GPS satellite cluster
10. Driving licenses valid all over the EU
11. Car insurance valid all over the EU
12. Pet passports that make travel with pets easy
13. Simplified fixed compensation scheme for flight delays & cancellations thanks to EU Air Passenger Rights.
14. European Health Insurance Card (EHIC)
15. Mobile roaming (calls, texts and data) at home prices
16. Portable streaming services (can watch Netflix etc. all over the EU)
17. Erasmus student exchange programme
18. Simplified VAT reverse charge mechanism for those selling across the EU
19. Safer food
20. Clean beaches
21. Enhanced consumer protection, including for cross-border shopping
22. Horizon 2020 (funding and assistance for over 10,000 collaborative research projects in the UK as part of the world's largest multinational research programme.)
23. Training courses for the unemployed funded by the European Social Fund
24. Disaster relief funding e.g. the 60 million euro we received for flood relief in 2017
25. Free movement for musicians and their instruments, bands and their equipment, artists and their materials etc.
26. Enhanced environmental protections
27. Court of last resort (ECJ)
28. REACH regulations & EU Chemicals Agency, improving human, animal & environmental safety around chemicals
29. Safer medicines thanks to pan-EU testing regime
30. Security cooperation and sharing of crime/terrorist databases
31. Participation in the European arrest warrant programme
32. EURATOM for medical isotopes
33. Support for rural areas ignored by successive UK Governments
34. Better food labelling
35. EU funding for the British film industry, theatre and music
36. European Capital of Culture programme, which has boosted cities such as Glasgow and Liverpool
37. Service providers (e.g. freelance translators) can offer their services to clients all over the EU
38. No UK VAT or duty on imports from the EU (great for online shopping
39. EU citizenship (it's a real thing with real benefits - look it up!)
40. Cross-border collaboration on taxes, to hold huge firms like Amazon and Facebook to account more than we otherwise could
41. Venture capital funding and startup loans
42. Legal protection for minority languages such as Welsh
43. Mutual recognition of academic qualifications
44. Legal protection for foods of geographic origin, e.g. Melton Mowbray pork pies
45. No credit and debit card surcharges
46. EU structural funding (eg. £2 billion to Liverpool) with matched private funding requirement
47. Supporting and encouraging democracy in post-communist countries
48. A bigger, stronger presence on the world stage
49. Use of EU queues at ports and airports
50. Products made or grown in the UK can be sold in 31 countries without type approval, customs duties, phytosanitary certificates etc.
51. Protection from GM food and chlorinated chicken
52. Objective 1 funding for deprived areas and regions
53. Financial services passport, enabling firms in the City to service the whole EU market
54. Strong intellectual property protections
55. University education in other EU countries at "home student" rates (many still have free universities.)
56. Mutual recognition of professional qualifications
57. Consular protection from any EU embassy outside the EU
58. Baseline of worker protections (which we can also improve on, of course)
59. The right to land fish in the EU (we send the EU half of all the fish we catch)
60. Enhanced medical research prospects
61. A friend to cosy up to against the might of the USA and China
62. Seasonal workforce to pick our fruit and vegetables
63. A vital source of medicines (we import 37 million packs a month from the EU)
64. Minimum 2 year guarantee on all products
65. Protection against unfair treatment in the workplace thanks to the EU Charter of Fundamental Rights
66. Minimum of 4 weeks paid leave (introduced by EU in 1993, taken up by the UK in 1998 and later extended to 28 days in 2009)
67. More influence on environmental measures that transcend borders (we get to help shape the policies that 28 countries must follow)
68. A say in the running of the EU (now we're outside, the rest of the EU carries on but we have zero influence over its policies)
69. Cleaner air thanks to the EU Air Quality Directive
70. Legally enforced 14 day cooling off period on timeshare agreements
71. Some of the highest toy safety standards in the world
72. Protection of 500 bird species under legislation dating back to 1979, and amended in 2009
It actually cost us about 40p per person per day.