How's this:
CPU:
http://www.newegg.ca/Product/Product.aspx?Item=N82E16819116775
Mobo:
http://www.newegg.ca/Product/Product.aspx?Item=N82E16813157297
HDD:
http://www.newegg.ca/Product/Product.aspx?Item=N82E16822236345
Case:
http://www.newegg.ca/Product/Product.aspx?Item=N82E1681114706
RAM:
http://www.newegg.ca/Product/Product.aspx?Item=N82E16820231445
PSU:
http://www.newegg.ca/Product/Product.aspx?Item=N82E16817139027
All those parts together should total a little over $500. It's difficult to put a PC together for $400, and almost impossible at $300. Of course, you could save some cash by transplanting old parts from other PCs, or buying used.
You could also save some cash on the above build by going with a Pentium, a lower end mobo, and a WD blue HDD. There's also some value memory, cases and power supplies out there, but I'd steer away.
You'd probably also want to add a sound card, but I know nothing about those. Expect another $50-100 for a decent model. You could also throw in a GPU if you were so inclined to do so, a GTX-650 would run you anywhere from $100-140.
I assume that you live in Canada considering that you asked for Newegg.ca links. If you live in Vancouver, Calgary, Edmonton or Winnipeg, Memory Express is a great local retailer. I picked up all of my parts there simply because they'd match online prices before shipping, and beat the difference by 25%, so you may want to consider buying locally. There's some great dealers with kick ass prices out there, I saved over $150 on my build.