Whenever one wants to buy a pistol it is best to go to the local gunstores and handle as many .45's as possible and see if it feels right. You will probably find that the newer polymer (plastic) framed pistols will be the cheapest to buy new although I doubt they will be $500 or less. If you find one in the $500-600 range and can't come to an agreeable price with your local gunstore then you may want to take a look online (example-www.gunbroker.com) and look up the make and model and compare prices. It's preferable if a new shooter buys his guns locally so the dealer can help you with any problems.
Buying used is a great option. I have bought about 15-20 guns used from online auctions (gunbroker or auctionarms) with no problems. However for a new shooter you may want to buy new as you may not be aware of what makes a good deal or how to smooth over minor problems with the used gun.
One pistol which has the same controls as your Beretta 92 but in .45 caliber is the Stoeger 45 cougar. They used to be made by Beretta until Beretta turned over their machinery to a quality gunmaker in Turkey. They are running $450-500 new on Gunbroker presently. I have one of the Beretta Cougars and they work great.
Good luck, shoot safely and make sure you save money for ammunition and training courses (very important).
best wishes- oldandslow