1. Should be able to run C++ programs
2. Price should be less then 20$ (better if <=10$)
It is not clear if you are looking for a microcontroller (i.e. a chip) with wifi support, or a single-board microcomputer with built-in wifi. If it is latter, then you have plenty of options. One of them is the latest iteration of the Raspberry Pi series of single-board systems:
The Raspberry Pi Zero W is available for $10. It contains on-board 802.11n wireless LAN and Bluetooth 4.0. You will have to add other stuff to get it going, but you should be able to run it headerless with nothing more than a 5V PSU and a 4GB micro SD card to hold the software.
One thing that I have not been able to establish is whether you can use the onboard Bluetooth to connect a wireless keyboard and mouse, or whether you will need to connect a wireless dongle to the (single) USB C socket, for which you will at least need a USB C to A converter cable. Starter kits are available, but this will increase the price considerably.
The Raspberry Pi 3 is more expensive than the Zero W (you should be able to buy one in the US for $35), but it gives you more connection options and it is what I would choose for development purposes. You get 802.11n and Bluetooth 4.1, plus 4 x USB 2.0 ports (with USB A sockets, so no converter cables), and also 100Mb/s Ethernet. Once you have got your software working on the Raspberry Pi 3, it should run quite happily on a Raspberry Pi Zero W.
Programming a Raspberry Pi using C/C++ is certainly possible, although most hobbyist use Python. The Raspberry Pi uses a version of Debian Linux called Raspbian, so a full stack of programming languages is available. Other operating systems are also available for the Raspberry Pi.
The CHIP is another possibility. For $9, you get onboard 802.11b/g/n, Bluetooth 4.0, and also 4GB of storage for software. It is not as well-established as the Raspberry Pi, and like the Raspberry Pi, you will need to add other stuff to get it going. However, you should be able to run it headerless with just a 5V PSU.
The important thing to consider with any of these systems is what you will need to add to make them usable, both for development purposes and also in the field.