I would look into using other wireless modules. XBees are nice, but are indeed expensive. I posit that you probably aren't using all of the options they provide in your projects anyways. Things like mesh networking are AMAZING, but aren't usually strictly necessary for most projects.
A very cost effective option, while still retaining some error checking, is to use IC's by Nordic, esp. the nRF24 series. There is an excellent arduino library for these modules that most likely can be ran on the Teensy with minimal adaption, especially if you are using the Teensyduino. This library also allows for some mesh communication (obviously not as robust as the XBees); see here (http://maniacbug.github.com/RF24/starping_relay_8pde-example.html).
If you order non-amplified versions of these modules, they can be had for under $7 per module. Here are some for $5.50 each.
Amplified versions are in the range of $15-$20, so if node to node distance is greater than about 40 ft the cost effectiveness compared to XBees goes down dramatically. It all depends on your usage.
From the same store that had the cheap Nordic modules, there are some cheap bluetooth modules that might be worth looking into; these have caveats in that they are meant to be connected to by a computer (a master unit) rather than each other (slave units). You can buy a master Bluetooth module for about $20 from ITeadStudio though.
Alternatively, the XBee CAN be used without a microcontroller for simple sensors and other uses. Then your cost is much lower per unit, around $25 instead of $25 + $16. See "Building Wireless Sensor Networks: with ZigBee, XBee, Arduino, and Processing" by O'Rielly for more details.