The ask price of DT/O Dragonite

Unown letter that I don't have and that was caught in my city.

Currently? Unown or Tyranitar with equivalent or superior IVs and a moveset that doesn't feature Fireblast. Probably no exceptions. I'd have to think hard on a Snorlax with 95.6%+ and legacy ZH/BS

$300

Gimme DB DC instead I love fast attacks but I'll never get one unless trading comes out /:

Due to the increased availability of Dratini candy, I don't think that mon will be in the first tier. This question, though, will likely be biome specific. So the answer is, it depends.

Lapras FB/ B or IB maxed out to my current level.

I have a spare 12/13/15 DT/O Dragonite Lvl 30.5 CP 3050 that's gathering dust in a stable gym, 50 Bucks.

I would probably trade one for a Body Slam Snorlax, but I've only got the one -- so I'd have to get another one first.

The price is driven by market supply vs demand. If bots and scanners exist during trading, it will drive down the market price to very low values... only a couple dollars or so. However, if they somehow eliminate mons caught using bots or scanners from trading, then the price would be tremendously higher. I already have a good DT+H Dragonite, so I wouldn't pay much. However, I'd love to have a legacy DB+DC Dragonite, which I prefer for dodging, and can no longer be found in the wild, so trade or never get.

To consider a Dragonite with high ivs and current best moveset "redundant", is a very high requirement for a legit player. That means he/she already has several lines of strong mons, to form an attacking team that is not much worse than the power of dragonites. Obviously you are not of this kind, since you are asking for such a dragonite. It is reasonable to infer that he/she generally has more/better metagame mons than you. So what mon are you willing to trade can attract that player? (I assume you do not have super rare mons like Unown)
Generally I think you have the chance if you would like to trade these mons with similar ivs:
Blissey without Psychic
Tyranitar with Bite+Stone Edge/Crunch
Snorlax with Zen+Body Slam/Hyper Beam
Dragonite with Dragon Breath/Dragon Claw
and perhaps Lapras with Frost Breath/Blizzard

I don't think you would have a team of either of these above(note the high iv requirement), which means the trade will not make you feel very worthy. The best option is to pay a spoofer $XX (Niantic will never be able to ban them all), and exchange it with a Pidgey.