i'm a novice myself but have gleaned a bit of information on the interwebs and from a friend who is an avid cyclist and works in a local bike shop.
$500 isn't entirely unreasonable but that really depends on what kind of riding you plan on doing and how much abuse the bike takes. cheaper bikes will still have some quality components, but i hear they will wear out quicker and require more maintenance than some of the higher-quality ones. i started off the way some probably have, by going to wally world and getting the GMC Denali for about $150. i didn't expect great things from that bike, and i got about what i expected, but it served its intended purpose and i realized after a few months that cycling was something i intended to keep doing.
after parts started to break on that bike, i picked up a barely used Forge CTS 1000 for fairly cheap and so far i've been impressed. the shifters and derailleurs took a bit of adjusting and still don't quite shift right from time to time, and the bike is a little on the small side for my 5'11" frame, but for the cost of the bike i'd say it's a good starter and if you take care of it then i don't see why it couldn't last 5 years. it retails online for around $400 ($380 at Target last time i saw) so that may be something you'd be interested in.
another option is buying a used bike. get on craigslist and start searching, there are some pretty good deals to be had on used bikes, i've seen some people list them for about half what they retail for just to get it out of their garage.
hopefully some veteran riders will chime in here, my knowledge is very limited and i'm currently in the market for a new bike myself but i'm looking at spending around $1000.