You can use any NAS as a Time Machine drive using the instructions contained in the link BTGeekBoy provided. I have an $80 1TB Lacie Network Space and it works just fine for Time Machine.
To make your NAS working with Time Machine:
In the terminal:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
One of the things you need is the hardware address of your network interface, to find, type in the terminal:
ifconfig en0 | grep ether | sed s/://g | sed s/ether//
Write down that number, as you need it for the next step.
To create the sparsebundle Time Machine needs for the backup, you type in the terminal:
sudo hdiutil create -size 100g -type SPARSEBUNDLE -nospotlight -volname "Backup of Kamin99" -fs "Case-sensitive Journaled HFS+" -verbose ./Kamin99Box_001ec4b8f9b3.sparsebundle
This will create a 100GB sparseimage for use with Time Machine. Kamin99Box is the hostname of your machine, and the number in Kamin99Box_001ec4b8f9b3.sparsebundle is the hardware address you got from the previous command.
You can now copy the sparsebundle you just created to your NAS, go into Time Machine and select your NAS, and sit back to watch it make your first network backup.