Here are two low cost options (I recommend #1 for most people and #2 if you have an impressive beard): 1. The esp8266 like you have already mentioned. Get one

Here are two low cost options (I recommend #1 for most people and #2 if you have an impressive beard): 1. The esp8266 like you have already mentioned. Get one with a u.fl connector and make your own cantenna to attach to it. You can use the microcontroller in the esp8266 to take the sensor readings (it even has an ADC to read analog sensors). This is the new hotness and there are tons of examples. 2. Even cheaper but less user friendly is a very low tech 433 MHz RF transmitter like you can get on eBay. You then need an external microcontroller to run it so you might actually be less cost effective overall and these things take quite a bit of effort to implement a reliable comm system because of the on-off keying system and zero built in error correction. There are libraries like VirtualWire or RadioHead that take quite a bit of pain away. You can get other frequencies if your country doesn't allow unlicensed 433MHz transmissions. Example of what I'm talking about: http://www.ebay.com/itm/like/140719918135?ul_noapp=true&chn=ps&lpid=82 http://www.ebay.com/itm/like/140719918135?ul_noapp=true&chn=...