I'm still running my Epson 4000 and 9800 in addition to my 9900.
My second-hand 9800 was charged with MK and it took a lot of ink to switch over to PK which it still uses today.
With a good profile, PK on matt paper isn't too bad, IMO and to most of my clients.
My rule of thumb is to figure that I'll use 2ml ink per square foot on any of my Epson printers. I came to to figure in printing many prints, figuring out how large the image area and then look up the job info regarding ink consumption for each of the prints. Most of the time, the 9900 uses quite less ink than my rule of thumb, but I prefer to error on the high side. My gut tells me that with cleanings, etc. I should probably add another .5ml per sf to the overhead.
I've switched a few times between PK and MK. I think the spec is perhaps 2-3ml for the change-over, but you'll need to figure a nozzle check and most likely a black ink pair cleaning once it's set. The change-over seems to be in the head and doesn't require the purging of the lines, just the head. Apparently, the pumps on the 9900 handle two channels each, so the change-over will consume not only the PK/MK, but also LK in the process. I think that changing from MK back to PK takes a little less ink.
Your cost per print will depend upon how much ink each will need. If you print high key mostly, it simply takes less ink than the proverbial black cat in the coal bin photo. If you have an idea of your ink consumption in printing your current 30x40 images, simply take that consumption factor and it should be similar to the 9900.
Your cost of ink will also vary depending upon the size of cart you purchase. The 700 ml carts will give you the lowest cost/ml, 150 ml carts, are the highest. However, don't simply buy a bunch of the large carts unless you are printing up a storm. Best is to get the smaller carts so that they don't "spoil" past the use-by dates! The recommendation is to use them within 6 months, but it's not set in stone.
When your carts drop to perhaps 30-40%, it's time to think of getting a replacement color since once they are down to less than 5-10%, you won't be able to do a cleaning. Once the cleaning is complete, simply replace the nearly empty carts and use them until they run out.
The last three Epson prints I've purchase second-hand and there have been few issues with any. The 7600 and 9800 were purchased on a whim and within months each had paid for its purchase and then some. The 9900 I bought for a canvas project since to upgrade the 9800 to the rotary cutter still required me to sit there and manual cut it. The 9900 handles the cutting of canvas automatically and is at least twice as past to print as is my 9800.
With the right images, the 9900 will get you a wider print gamut but the 9800 isn't a sloucher in that department either!
If you are using any cut sheets for printing or need to output cut sheets in quantity, hold on to your 4800. Feeding one sheet at a time in the 9900, especially smaller than 11x17 takes time and a lot of handling.
As to a new printer from Epson, I couldn't care less. I have what I need and hope to simply wear them out. At that time the 9900 will require 4 pall bearers to remove its carcass from my studio, the same number it took to wrestle it down the stairway and into the studio two years ago.
We ran my Epson 2200 into the ground a couple of years ago. Had it since new and after 6,000 prints twice, the tech said it was time to make it a parts donor. It got replaced with the 3880 and that should last another ten or so years.
Get what you need today and run it until you wear it out or can't find the inks any more.