I think you're getting mixed up.
The amount varies but most recently was £18b. The rebate is applied before that money is paid, which in 2015 was £5b so the amount paid was £13b. You then factor in the EU funding provided to us, which was calculated at £4.5b in 2015. This means the net contribution was around £8.5b and nothing like £20b.
These figures don't include payments made to private sector areas, like scientific and medical research, so in reality this figure is still lower but it's not easy to quantify those amounts so I didn't put them in the calculations.
None of this, of course, counts the private sector businesses that trade within the EU, paying taxes on their earnings directly out of the EU, nor the services that the EU have covered off for us - the civil service is going to have to grow at hideous expense to cover off the shortfall if we leave.
The actual cost is tiny compared to our GDP and the overall impact. All of this information is freely available. Here's a link:
https://fullfact.org/europe/our-eu-membership-fee-55-million/