tl:dr version: I'd plan on not much less than $10k.
Tricky to really answer. They may be bigger, but cannon are like most guns on the market. Prices and quality can vary widely.
When you say "new", I presume you mean modern made and complete enough to fire. I also presume you mean full scale, not miniature. I would expect to pay about $10k, maybe 9 for such. Now, that would likely be "used" but recently made. One of the more common examples of rrepro rifled artillery that actually is rifled would be a 3" Parrott rifle made out of a naval 3" gun barrel on a proper field carriage, and many of these are pretty good. Oh, and there's a place in Idaho that was making 1841 12lb howitzer barrels out of surplus 105mm howitzer barrel sections. Historically odd, but pretty cool, and that guy knew what he was doing in terms of safety and metal fab. No steel rifled 12lb howitzers ever existed, but they looked great and I wouldn't have been worried about pulling the lanyard on one from a standpoint of safety. Reenactors, much maligned and often times rightly so, are not a horrible resource for reproduction artillery. Many of those guys are actually very knowledgeable, and, since they have a use for old style artillery but people leave that hobby, they often know of cannons for sale that may not be advertised anywhere.
If you mean actually brand new, like "still has the tags on it" new, the barrel would probably be about $8k and up, potentially WAY up depending on what you want, and the carriage would cost about $7-8k IF you mean a field carriage. If you just want to fire a cannon at a target a ways off and have firm level ground from which to fire the gun, a navy type carriage, actually I think the term is "truck" if the navy guys are correct, could probably be used and would cost quite a bit less. A Parrott or ordnance rifle would be historically dubious mounted to such, I don't think the navy used the smaller rifles much if at all, but it could be workable. Off the top of my head, though, I don't know of a source for that type of carriage. Good ones, that is, not just decorative. Naval gunnery generally isn't something I know a great deal about, I'm afraid.
And with either carriage your new with the tags piece would require a ram/sponge, a thumbstall, a vent pick, a lanyard (probably) and a worm. Oh, and a mandrel of some sort about 1/4" under bore size to form cartridges with, ammo being a topic unto itself I won't get into that here. Suffice to say most folks make them out of foil and you'll need a mandrel to do it. Figure about $400 total for those. You'd also need those if your used but recently made cannon didn't include them.
You'd do well to have a box of some sort to hold implements and ammunition. Depending on your plan it may not be strictly necessary, though. An authentic limber figure $3k, but again, depending on your goal, a surplus 105mm ammo box can work well enough and those don't cost any 3 grand. They also aren't terribly waterproof but can be made to be, if that matters to you.
You'll also want to decide HOW you're going to fire it. Friction primers, the repros of which had earned my disdain but do seem to have improved recently, are I THINK about $5 a piece. I threw in the towel on authenticity on all of mine and installed a cannon lock (about $30 at Dixie gun works) and had vent liners installed along with them (machinist friend turned them out for me for free, but they resemble and can be made from a bored out hex bolt of appropriate length and strength) and fire all mine with 209 shotgun primers. For either of those, you'll need the above mentioned "probably" lanyard. I don't want to get into quills and slowmatch because they're more confusing than what I've already spoken of and because I don't like them anyway. Or, finally, you COULD just use green fuse if this is something you're interested in for just yourself mostly and not because you're giving spot-on demos of exactly how they used to do it backinnaday.
There may well be some finer points and/or details I've left out or forgotten, and if you've any by all means feel free to ask.
If you're serious about this, or even if you're just interested in older cannons generally, as others have mentioned get to the GBO cannon forum. HUGE source of knowledge, as well as sometimes conflicting opinions. lol
Cpt. Redleg