All but a few pickups for acoustic guitars fall into one of two catagories, magnetic, and piezo. Magnetics need steel strings, so we can ignore them for your purposes.
The most popular piezo pickup is the Fishman Matrix Natural I. This is probably what I would recomend for you. It is an undersaddle pickup, which have the most consistant sound and the best resistance for feedback. The pickup consists of an undersaddle pickup element, a preamp, and a battery clip. The battery clip gets mounted to the neck block, the preamp is a part of the new end pin jack, and the pickup gets mounted underneath the saddle. The is one small hole in the bridge and top for the wire from the pickup to the preamp (about 3/32", so tiny), and the endpin jack hole. There is no perceptable change in the sound of the guitar. Total cost, including professional instalation and the pickup, runs about $235 in my shop, which is probably a fairly acurate median price for the job.
Other options include similar systems by L.R. Baggs, Highlander, and D-Tar. I beleve B-Band is also making a similar system. The Fishman is by far the most popular of these systems, which all sound fairly similar, but the Fishman is much easier to install, and is therefore cheaper to install. Fishman is also the pickup of choice of most of the major manufacturers, including Martin, Gibson, and untill very recently, Taylor. Taylor still use Fishman pickups for their (really crappy) nylon strings, I believe.
Other possiblities inclued: the Baggs iBeam, which is a similar system, except the pickup is mounted in a "beam" under the bridge - This would not work, however, on most clasicals, as fan bracing does not provide a flat mounting location for the pickup; the Acoustic Feather (I can't remember who makes it right now), a pickup which does not impress me at all; or some varity of internal microphone. Internal mics, on the whole, do not sound all that good to me, and they are prone to feedback.
The other possibility is a clip on mic, which is what many classical guitarist seem to prefer, as it comes closest to the natural sound of the instrument, but I find them to be overly prone to feedback, and usually overly bassy. If you are only needing a small amount of amplification, though, they can be aceptable.
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