"I have a 12" ported backfiring enclosure and was thinking on getting a driver and amp plate."
the size and tuning of the enclosure affect the performance once you plop a driver in the box.
"However, I noticed that people are recommending an external amp. Is the amp necessary if I have a subwoofer plate?"
if you have a plate amp, you do not need another one.
"Also I noticed that the subwoofer plates are extremely expensive...somewhere close to 200 bucks. Is that correct?"
they can actually cost over a thousand dollars. as for if that is expensive, that is up to each person to decide.
"I might as well buy a new subwoofer if I have to spend 500 for a 500watts amp. Then why build your own."
because you might like the performance of something that you cannot buy for the same money. if you want a cheap sub tuned to 33hz or so, then you are probably better off buying.