In general, the brake-conversion-kit market is geared toward high-performance -- not to mention high-cost -- brakes. So deciding whether to buy a conversion kit or gather parts from a junkyard or parts store depends largely on what you're looking to get out of your conversion.
If you're simply upgrading brakes on a much older vehicle, a conversion kit might be overkill, and could end up costing a bit more than donor parts.
On the other hand, the nice thing about conversion kits is that they save you the time and effort of hunting through junkyard wrecks and auto store parts bins to find what you need. For instance, a conversion kit for a 1970 Ford Mustang costs between $1,000 and $2,000. The same conversion performed using a combination of junkyard and auto store parts, cost one do-it-yourselfer $800. [source: Ireland]
Conversion kits can cost anywhere from $500 to $3,000. The lower-end conversion kits have simple flat rotors, while high-end kits often have rotors that are slotted to keep them cool. High-end performance kits also put emphasis on showiness, offering special finishes for the rotors and bright red racing-style calipers.
Keep in mind that although there may not be a specific conversion kit available for your vehicle, there's bound to be a compatible kit out there. Car enthusiast Web sites, forums and message boards are invaluable resources for finding out what kit best fits your needs for a conversion.