A rule of thumb for me since the 1970's:
A brand new (but readily in supply) firearm loses ~15% of its out-the-door price when you walk out the door.
After that, an otherwise mechanically perfect, but fired weapon, with a little honest fair wear (maybe a little finish wear on holster contact surfaces such as muzzle, cylinder, or sights) might bring 70-80% of full retail value.
Example: Glock 19 retails out the door for $600 today (base price, taxes, and background check). Tomorrow (unfired) it's worth ~$500-$525 in a face to face sale. After a year of occasional range use and a bit of concealed carry...it's worth ~ $420 in that same private sale or trade scenario.
Naturally, laws of supply and demand apply for harder to find and more desirable specimens. You could ask for (and get) whatever you have invested in a Colt Python .357 or Smith & Wesson M58 .41 magnum (unless you overpaid to begin with).
I look for and buy a lot of nice used handguns in the $300-$500 bandwidth. This price range includes a lot of Colt, Smith, Dan Wesson, and Ruger revolvers. It also includes most pocket, compact, and duty sized semi-autos (BHPs, S&W, Colts, Glocks, CZs, etc.).
Some brands are pretty high priced at retail price point to begin with. HKs come to mind. Not so common and folks are loath to part with them for a lot less than they paid (unless they are in dire financial straits).
Some brands and makes are so plentiful on the used gun market (e.g., shot guns like the Mossberg 500 & Remington 870), that they can invariably be found used for about 40% off retail price with a little searching. A common $300 package shotgun (readily available at WalMart) becomes a $175-$200 shotgun in a private trade or sale.
Just because someone payed $1200 dollars a few years ago for a higher end Kimber 1911 does not mean that the weapon still holds said value. $850 might be more like it.
The presence or absence of all original factory accessories (original grips, box/case, manual, cleaning tools, magazines) or the inclusion of useful owner accessories (quality holster, better grips, extra mags, speed loaders, or ammunition) is important. These accessories will justify a slight increase in the asking price (or can be used to sweeten the pot on a trade). Take off about 35-40% of the aggregate value of all additional accessories or ammunition. Nine times out of ten, the seller has little use for the extra weapon specific grips, holster, or ammo after he/she sells the firearm. Add that value into the amount you are prepared to pay for just the weapon.
For instance: Seller has a nice used Glock 19 (with factory box and accessories) that he wants $450 for (a little high in my mind...I'm thinking more like $350-400). He'll also include two boxes of ammo (1 x 50 rd box of FMJ practice ammo and 1 x box of premium hollowpoints = about $50 retail worth of ammo). He also has a $80 dollar leather holster from a name company (showing very little use). He also has 2 x extra used factory magazines (worth about $40 total). $50 + $80 + $40 = $170 retail value for these items. Subtract 40% of $170 ($68). $170 minus $68 = the extra items are worth about $100 dollars to me.
He wants $450 for everything. I want to pay $350 (if I can). The extras are worth an additional $100. It's a fair deal (unless I can talk him down a bit more). I'll offer $400 and show cash. Don't be afraid to haggle.
If a seller wants $20 less than full new retail for his used weapon plus he adds in the price of all the additional accessories he invested in...walk away.
Or just give him your cash offer. Some folks start off with a high asking price in the hopes that an uninformed someone will pay. They always have a mental price floor that is much lower. Cash talks. Haggle. It's fun and doesn't have to be insulting for either party.
For an otherwise perfect used weapon: Without the factory original grips, magazines, box/case, owners manual, and factory accessories, I'll drop about 20% off the value I'm prepared to offer.
Hope this helps...
EDIT: You didn't specify which model of Beretta you were looking at (either the .380 ACP or .22 Long Rifle flavors of the Cheetah). In any event, $425-$500 is a reasonable price range for a used example in excellent shape.