What kind of BT “protocol” will you need?
If it’s something simple like SPP only, you can get modules like the HC-06 for less than $2.00 from China.
If you want BLE a Particle companion produces the RedBear Duo which has both radios on-board and uses the Particle infrastructure.
BTW: Where does “I did not expect $100 just for Wifi.” come from? What device have you been looking at?
There's no 'X10' module. You're talking about the P0, where you get 10 pieces (x10) for $100, so that's $10/each. Unless you like playing around with RF circuits and designing your own antennas, you might want to look into the P1, which had this integrated already. It's also certified, which makes life a bit easier.
Alternatively, there's the Duo that @ScruffR mentioned that had WiFi and Bluetooth standard. Might make things easier as well.