If you have two Arduino's and you want to go cheap (only 9$,and 3$ on ebay)! Use the RX communication. These are good for beginners.
https://www.sparkfun.com/products/10534 - RF Link Transmitter (434MHz) from SparkFun for about 4$
RF Link Receiver - 4800bps (434MHz) - WRL-10532 - SparkFun Electronics - RF Link Receiver (434MHz) from SparkFun for about 5$
http://www.ebay.com/itm/315Mhz-WL-RF-transmitter-and-receiver-link-kit-for-Arduino-ARM-MCU-/370776247168?pt=LH_DefaultDomain_0&hash=item5653fc8b80
Ebay Just $1.50 a pair!
Unfortunately, I don't have two Arduino's so I've chosen a harder road: Communicating between an Attiny85 and an Arduino UNO. I created a similar topic for this so if you interested go look at it as well.
http://arduino.cc/forum/index.php/topic,154684.0.html
But yeah, if you have two Arduinos communicating, I guess it would be cheaper and much simpler. XBee's are good options but they are hard to configure (according to other peoples views, not mine) and pretty darn expensive!. But personally I think RX communications is much simpler. Plus the RX antena's have a amazing range if you add a simple additional antenna to them (consisting from just a wire that picks up signals).