The Omni is an omni-directional, small-diaphragm condenser microphone that can be built at a very low cost.  The creator offers detailed instructions and a bill of materials on his website.

Notable Features:

• Small-diaphragm capsule from Projects Unlimited
• Omni-directional pickup pattern
• Electret condenser type

Price: $7 for instructions and bill of materials

Adjusted cost: $25 (estimate from HomeMadeMicrophones.com)

More information at www.homemademicrophones.com.