I've never seen anything more than anecdotal information about something as basic as the breakdown between APS-C (plus Micro 4/3) and full-frame. Anecdotally, full-frame is something between 10-20% of the market...
What would be more interesting is actually divided something like this:
Entry-level - All Rebels plus D3x00, D5x00, a5100,a6000(?) X-T100, etc. Anything under $800 with a kit lens. APS-C and Micro 4/3 options. Very few cameras in this class are ever used with a lens they weren't purchased with (some come with two lenses)
Lower-end enthusiast - D7x00, EOS-x0D, D610,EOS 6D, A7II, a6500, X-T2, E-M5 mkII, etc. (at current pricing) All three format size options, although full-frame will be an older body paired with a notably low-end lens to get this low (you can get some excellent APS-C and Micro 4/3 lenses in this price range). Anything between $800 and $1500 with a kit or "super-kit" lens. Many photographers buy an additional lens, often a telephoto zoom, sometimes something wide (or both) to go with this sort of camera - often after the body purchase.
Upper-end enthusiast - A7III, X-H1, E-M1 mkII, D750, Pentax K1 mkII, D500, etc... All three format size options - the top end of APS-C and Micro 4/3 mixes with lower-end full-frame choices. Anything between $1500 and $2200 with an initial lens (which may or may not be bundled with the body). Most photographers own other lens(es) to go with their system, and many have investments in flashes or other accessories.
Prosumer - EOS 5D series, EOS-R, A7rII , Z6, etc. Anything under $3000 for the body... You sometimes get a promotional price on a lens with the body - but it's often a "pick a lens" deal, because most photographers at this level will already have a normal zoom they like (although they may pick up a second to have one for each body, to upgrade to a new version, or this may be a first full-frame body). All full-frame
Professional (fast) - Three possibilities - EOS 1Dx mkII, D5, A9. Anything over $3000 for the body with a top speed over 10 FPS, the finest AF in the world and a penchant for long glass... All full-frame (although Olympus is planning a Micro 4/3 body in this range!). Most photographers who use these own a closetful of lenses, some of them worth more than their car...
Professional (pixel monster) - Z7, D850, A7rIII, EOS 5Ds/5DsR, GFX 50R and 50S, X1D. Anything over $3000 for the body with a top speed slower than most $1000 cameras and a file size that crushes hard drives. Full frame or larger. Most photographers who use these own a closetful of lenses, often with unpronounceable names, and a printer that pretends to be furniture...
Exotic - Leica, Phase One, Hasselblad H System, RED, 4x5 film, etc. Just how small a piece of the market is this? Everything from weird $3000 compacts to $50,000 Phase backs...
Of course, these categories aren't perfect (there is no place to draw the lines that won't put some camera in a category different from a close relative). The weird divisions here are that the X-T2 ends up a category below the X-T3, and previous model Sonys end up in different categories from their successors. Neither of these seem right, but splitting some Sonys is inevitable in any scheme because of the profusion of previous models, and if the Fujis hadn't gotten split, something else that shouldn't have would have...
Any scheme like this (or model by model sales data) would tell us much more about what's going on than we know today.
The only solid numbers (not percentages) I've seen are that ~11 million total interchangeable lens cameras are sold annually (Thom Hogan among other places), and that Nikon can make up to 20,000 Z-series bodies (Z6 and Z7 combined) plus 10,000 D850s per month (a couple of Imaging Resource interviews). Assuming that those three models are half of Nikon's full-frame production (I suspect they are somewhat less - the less expensive D610 and D750 probably sell more, even though they are older models), Nikon alone sells at least ~700,000 FF cameras annually. If Canon and Sony are similar (most estimates I've seen are that the full-frame market has three similar-scale players), and Pentax and others are negligible, that suggests that at least a couple million, and maybe more like 3 million of the 11 million cameras sold each year are full-frame.
I saw an estimate somewhere a couple years ago that Fuji sells roughly half a million cameras annually, mostly higher-end models (this was before the X-A series really took off in Asia). Throw in sundry E-M1 mkIIs, D500s, EOS-D80s, a6500s and the like, plus a few (probably less than 100,000/year, even with Fuji's expansion of the market) medium format bodies and my best guess is that the enthusiast and professional camera market is somewhere between 3 and 4 million bodies annually.
This is very rough, but it's interesting - there's almost certainly more revenue (not to mention more profit) outside the low end than there is in the low end. Even if the split is 8 million entry-level bodies and 3 million of everything else (and I suspect it's closer to 7 million/4 million, depending on where you draw the line), the non entry-level bodies would only have to average something like 2.5 times as expensive before they became a majority of revenue. If it's 7/4, they could be less than twice as expensive... This is before including the effects of lenses - nobody buys a 400mm f2.8 for their D3500!