Probably about the same as the official supply, you'll probably start seeing them pop up. The current price I can see for the 5V5A supply is £11.90. They go i

Probably about the same as the official supply, you'll probably start seeing them pop up. The current price I can see for the 5V5A supply is £11.90. They go into the trade-off in one of the launch videos. Apparently if you've only got a 3A supply then the downstream USB power gets limited but otherwise it works.