Need a cheap embedded linux board that has WiFi access point and USB2.0
I've had some success with routers running OpenWRT as embedded platforms. My router of choice at the moment is TP-Link's TL-703N. This is a Chinese device shipped with a Chinese web GUI but there are great instructions on the OpenWRT wiki for installing OpenWRT on the device.

The device is sold as a travel router for 3G dongles. It doesn't have 3G built in but does have 802.11n WIFI, ethernet and USB 2.0. Its main feature is price, I got mine from eBay for £17 (about US$25).

I wrote up a wee post on the device on my hackerspace's blog, here.

It's a nice, small, embedded platform, and it's possible to tack some wires on the board for a hardware serial port.