I've had nRF24L01+ow power modules transmitting about 30ft through a couple of walls and a floor. In open space I think they'd work fine.
For about £1.30 each from China, it's certainly affordable to get yourself a few and give them a go with the Maniacbig libraries. RF24Network was designed as a sensor network so should do exactly what you have in price.
If you do need longer range then the high power version of nRF24L01+ is available for about £7.50. I've had them doing about 150m through a window and down a street in an urban environment. If you can get height to clear the Fresnel Zone better then I'm sure you could increase that.
With RF24Network you can insert repeater nodes to extend range, in a similar topology to XBee but at a fraction of the price.