$12,000 to $15,000 is a registered Lightening Link. If you see an RDIAS for 12-15k, it is almost with 99.99% certainty, a scam.
The last three RDIAS sold on gunbroker for $29,000, $30,000, and $31,000, respectively. I recently just bought a steel Broadhead armory RDIAS (which IMO is the best RDIAS). If you skip the broker, you can find a RDIAS for $26,000-$28,000 or so, but you're dealing with an unknown person, and run the risk of being scammed. I would not advise buying an NFA item directly from a person/individual until you are more experienced in the NFA realm.
The RDIAS can come in either steel bodied or aluminum bodied. The trip, which is not the registered part, is always steel regardless of the body. An aluminum bodied RDIAS is about $500-$1000 cheaper than a steel RDIAS. Some would say that there is no difference between a steel and aluminum RDIAS. I prefer steel, as I am pretty sure that while a kaboom may not destroy the aluminum RDIAS, the aluminum RDIAS is more likely subject to deformation or wearing down by being rubbed against the gun well.
I own most of the commonly available variants of the full auto M16/AR15, except a RLL. My favorite is the RDIAS, of which I own several. I absolutely love them, and can't say no to buying a reasonably priced RDIAS. The M16 is cool because you have something that says "M16", but the RDIAS is much more versatile. With the RDIAS, you can have a new host, new gun, and configure it however you want. Also, if you have a kaboom that destroys the gun, you can simply swap out hosts, as the RDIAS is nearly indestructible. There isn't much downside to an RDIAS, as it isn't even the most expensive option.
The only real downside to an RDIAS is that only one or two are available for sale, at any time, across the entire country. You will be competing with other buyers, to buy one at a reasonable price.
But to answer your question, as of this writing, here are my opinions as to current RETAIL prices, with the low price being a "deal", and the high price being "wishful thinking from a real seller". This is not an exhaustive list of all options. The average is in the middle:
M16A1 complete gun: $21,000-$26,000.
M16A1 lower only: $20,000-$25,000.
M16A2 "burst" variant marked: $28,000-$33,000.
M16A2 "auto" variant marked: $30,000-$35,000.
M16A2 carbine: $30,000-$35,000.
M16A2 marked with "Property of US Government": $35,000-$45,000, with $39,000 average. Note: be very careful buying these "Property of US Government" guns. Many were actually Stemples that were put together and not actually genuine "Property of US Government" guns. Buy them from a reputable dealer AND have the dealer take apart the gun and have them look over ALL the gun for any markings that may indicate the maker of the gun. For example, on a Stemple "Property of US Government" gun, you can usually find Stemples' markings on the area that is covered by the pistol grip. You pay $40,000 for a gun thinking it is a genuine "Property of US Government" gun, then 30 years later, your heirs find that it is a reweld when a knowledgeable buyer inspects it. DOAH!
There are other variants, but the above prices are for the commonly available.
AR15 Colt receiver or complete gun: $14,000-$20,000, average of $17,000. If you go with this option or the "off brand" option below, I would have the seller or you check the take down pins and ensure that they are standard pins, otherwise, you won't be able to use most uppers.
"Off brand" AR15, like EA or Bushmaster: $12,000-$18,000. These are known as shooter ARs, as you can shoot the hell out of them, and then just replace the upper. This is the true entry level full auto AR gun. You're just buying the lower, as the upper is worthless.
"Billet" registered Machine gun lower receiver: $500-1000 above the "off brand" above. You sometimes see these for sale at a $500-$1000 premium to the off brand AR15s. Not worth it, IMO, as no one, but the seller, really cares that it is a "billet".
Registered Lightening links: $10,000-$15,000. Many have been repaired, as the paddles on the RLL that acts as the trip will eventually break. I would never buy one of these.
RDIAS (Registered Drop in Auto Sear) Steel: $22,000-$32,000. Anything below $25,000 will sell the same day (however, if it is below $25,000, it is almost always a scam), $26,000-$28,000 will sell in a week or a month, and $28,000+ will take over a month to sell.
RDIAS that is symmetrical on both sides and steel, like a Broadhead Armory RDIAS, will be $30,000+ at retail, and will sell in like two days. You may spend your entire life looking for one of these.
RDIAS aluminum body: $500-$1000 less than steel. Some would argue that the aluminum RDIAS are really no different from the steel RDIAS. Functionally, that is true.
The DIAS that ATF is confiscating are the pre 1981 RDIAS, not the registered DIAS that are on the books, tax paid, with a form 4, and registered prior to 1986.