Has the exact cost of any of Iron Man's armors ever been mentioned in any continuity?
Guess what, it depends
Seriously though, 50+ years of Iron Man comics/movies/series have introduced a good bunch of armors, so there is no definite number.
However, here's a list of "Tony Stark implies armor X cost Y dollars" references, by chronological order. I'm excluding AIs, villas, space stations, etc.
Iron Manual armor (1993): $1.8 million a glove
Iron Manual (1993) lists pretty much all components in Iron Man's armor, but surprisingly, only one actual cost is given, that of a glove. $1.8 million for a single glove; up to the reader to extrapolate how much that elevates to for the whole armor.
Turns out the beam irregularity was due to fluke "environmental interference" - make that a speck of dirt. Good, as I hate replacing these gloves. Not that I'm a penny-pincher, but each one costs $1.8 million (1988 adjusted dollars). Not including labor.
Iron Man Dio Armor (2010): $1.5 billion
In the 2010 Iron Man series which was part of the Marvel Anime shows, Tony Stark aims to mass-produce Iron Man Dio Armors. The prototype presented in the first episode (timestamp: 9'34) costs around 1.5 billion dollars, based on what he says to the Japanese Minister of Defense:
This is the perfect time, Minister, for that favor I asked you the other day. Let's find out what one point five billion dollars can do, shall we?
Godkiller Mark II (2018): $4 billion
Built "just in case" heroes had to stand up against Celestials, and kept parked on Mars, because yes, Tony is that rich. It's a bit of cheating, since it's Celestial-sized, but Tony does control it from the inside (while wearing another armor), and eventually lets it blow up in Avengers #6 (2018), hoping that will get the Avengers rid of at least one Dark Celestial.
Those are four billion dollars' worth of fireworks. Never let 'em say we died without flair.