Quite a few $30 - $60 webcams have better audio than the overpriced and gimmicky supposed-high-quality microphones that are advertised for $100 - $300.
A large majority of those just have standard cheap electret condenser capsules (similar to those in mikes sold for $1 - $20) but with "electret" omitted from the description to get people to confuse them with true condenser mikes.
A decent gaming headset microphone is probably similar quality.
Or, try something like this:
http://www.ebay.co.uk/itm/eBerry-Hi-Speed-USB-2-0-Computer-Microphone-Mic-with-Bendable-Goose-Neck-and-/282301278163?hash=item41ba77a7d3:g:WpsAAOSwux5YXYjp
It's a standard electret mike, with USB interface, at a sensible price.
If you want seriously high quality gear, then either a Shure SM58 or MXL V67g microphone plus a USB Audio interface, eg.
http://www.ebay.co.uk/itm/Shure-SM58-LC-Vocal-Microphone-Cardioid-NEW-/272481526954?hash=item3f712a24aa:g:r8MAAOSwo4pYTGQt
http://www.ebay.co.uk/itm/MXL-Marshall-V67G-Microphone-Brand-New-/181772208528?hash=item2a5277c990:g:4HgAAOSwYIhWj0MQ