APPLE A1347 MC270LL/A Mac mini 4.1 Core 2 Duo 2.40Ghz 4GB 320GB - HDMI
This particular model ships with 4GB of RAM which you can double to 8GB for about $50.00
The hard drive is an okay size at 320GB but you could add a 250GB SSD drive for about $100.00 to make this mac mini much faster than if you were to use the standard drive.
Total Upgrade Price for APPLE A1347 MC270LL/A Mac mini 4.1 Core 2 Duo 2.40Ghz 4GB 320GB
If you add the cost of the Mac Mini and double the RAM ot 8GB and swap out the drive to an SSD drive you should be able to spend less than $400 for a Mac Mini that still has lots of life left.