Imagine a serial device like a drawing tablet connected to an RF
transmitter. The transmitter and the drawing tablet are powered by a
9V battery. Meanwhile, about 30 ft. away, there's a receiver hooked to
an rs232 port on a computer. 9600bps is all that's required for speed.
How can I make this work cheaply?

Why do you need serial device, you can do it with USB,soon wireless
USB would be out in the market, also bluetooth support is available at
present for this purpose, bluetooth devices operating at 2.4 ghz will
do

That would be throwing money at the problem instead of answering the
question, which is how to build a specific type of device.

I have an existing serial device and I need to make it wireless over a
short range. Part of the criteria for the conversion is its cheapness;
RF packages that transmit at 9600bps are cheap compared to a Bluetooth
or UWB/WUSB device.

I would imagine if you buy a serial device from eBay such as a drawing
tablet or mouse it would cost maybe 10% of a new USB/BlueToothdevice.
There are also existing devices that convert rs232 to wireless and
back again, but they are in the hundreds of dollars range. I want a
package I can build for less than $20 because I'm asking for so
little--transmitting digital data at 9600bps over a range of 30 feet.

I have this and it works great:

http://www.iogear.com/main.php?loc=product&Item=GBS301&name=Serial%20Adapter%20with%20Bluetooth%20wireless%20technology

Thanks, but $100 isn't less than $20. I could buy a whole new device
with built-in Bluetooth for $100. What I'm looking for are plans
online on how to build a low-range digital transmitter/receiver pair
that works with ttl logic (rs232)...does anything like that exist?

Give this a look...
http://www.hobbyengineering.com/H2248.html

Not $20 but closer to what you are requesting.

Sorry, here is the companion device...
http://www.hobbyengineering.com/H2247.html

Apparantly there is no dirt cheap device as you've described already
available on the market.  Any number of electrical engineers could
design you a simple solution to your problem, however, even if the
cost of the parts for such a device could be kept under $20, the time
and effort to design such a device would be worth much more than your
meager $2 offering.

Wireless USB is currently around $20, and going down.

Bluetooth is ~$4, and more-or-less floored out.  You can buy a
bluetooth module with embedded "serial-port-profile" (effectively a
raw L2cap link) from most of the BT vendors.  Add a couple lines of
code for forced-enumeration, and you've got a sub-$10 solution for
point-to-point RS232.  Alternatively, you could directly modulate an
RF oscillator with the RS232 signal and run it into an FM )(or AM
receiver).  Downside is no error correction and more NRE.

drm