Seeking cheap RFID reader / writer
There are a whole bunch of different RFID types, so it's hard to recommend something without any further information.

I'll assume that you meant EM4100 125kHz tags, those are read only, but very common, so there are many choices.

If all you want is to get the rfid ids into a pc, then pick up an USB reader, which simply shows up as a USB keyboard, which spits out the id and hits return:
http://cgi.ebay.com/125Khz-RFID-Proximity-ID-USB-Reader-Free-5-RFID-Cards-/260641368732?pt=LH_DefaultDomain_0&amp;hash=item3caf6faa9c

For your own electronics, the cheapest choice is to hook about $2 worth of jellybean components up to an AVR, like I've done here:
http://www.hackaarhus.dk/forum/download/file.php?id=45
Forum thread: http://www.hackaarhus.dk/forum/viewtopic.php?f=4&amp;t=113
Sources: https://github.com/dren-dk/HAL900/tree/master/door-ctrl/hardware-4

Next step up is a module like this:
http://www.sparkfun.com/products/8419