1. Humidity sensors distributed transmit and receive data wirelessly. (~ 5 meters Arduino)
I wonder if you can just use an Arduino for the project
if so, what should I use the wireless transmitter system.
ps: Which model Arduino compatible to the project?
How often? What is in between the sender and the receiver? What is your budget? How critical is it to not lose a reading?
As for "Transmitting always" that is not feasible. Are you transmitting every 1ms, ever 10ms? every 1s? every 1 min? You cannot always be transmitting, especially if you intend to go off battery, and transmissions cost a lot of energy. How often do you need to know when your sensors have new data? Is there any reason you cannot just transmit every 15minutes? Is your humidity changing that rapidly?
$120 is going to limit you, at least depending on the number of sensors you plan on building at least. If your Xbee costs $23, and your Arduino costs $25, that's almost half your budget for one of the sensors, and that doesn't include the sensor.
OP: How many senders will there be?
But, if using XBees, there doesn't need to be an Arduino at each sender
One XBee can collect data from several sensors humidity?