Regarding operating expenses, it depends on how you're going to use the MKVI, or perhaps any rebreather. Admittedly, startup expenses do vary, but the MKVI can easily be purchased for $6,800 or less, and the training is averaging about $750 plus PADI materials. The training can escalate if you want the advanced class which includes the use of bailout gasses and allows deeper depth.
Your earlier concerns assume you'll be purchasing a MKVI straight away. According to your profile you're in Fort Lauderdale which places you reasonably close to Silent World, who has units on the shelf available for diving - today. In addition, right after DEMA, SeaExperience's owner reported a serious interested in placing two of them in his rental line after the remodeled Fort Lauderdale shop is completed this January. This is all positive information for you as you can get the training, and rent the units with relative ease right in your backyard before deciding to purchase.
Per dive cost somewhat depends on what exactly you're going to do. Afterall, if you use one liter of oxygen per minute, well that's what you use. If you're already paying $12 for an airfill, you're actually paying more per dive in gas cost by a significant margin. My two 13 cu/ft cylinders generally give me four one hour dives on a single fill (no drysuit). Assume you pay $48 (4 fills X $12) for a day of Open Circuit Diving in a standard Morning 2-Tank and Afternoon 2-Tank. If I take my CCR along to be your dive buddy, I'm only paying a gas bill of $12.32 plus Florida tax for the whole day of diving (using FillExpress.com's current posted retail gas rates).
Now, let's address the issue of the scrubber. I have spent $13.00 in gas for the four dives, and now I have to buy a $33 scrubber so I'm in it for $46 plus tax. The scrubber appears a non-issue to me.
Now being slighly more practical as the above is not representative of how I actually dive my CCR. With the limited cost of adding 4 cu/ft of helium to the diluient, I almost always run a 21/35 diluient instead of air. By choosing this diluient and assuming I manage it properly, I shorten my deco time, significantly decrease gas narcosis, and can go to the gas depth limit without any [measurable] additional cost per dive. I personally believe it's important to rid myself of any narcosis on the rebreather as staying sharp is necessary, so again I use Trimix. Using FillExpress.com retail rates, I calculated the gas fill for four Steel 100s of 21/35 at $160 to make the equivalent OC dives on 21/35, and I'm not even bothering to calculate the OC deco gas because we'd need SAC rates, dive profiles, etc. What's really important is that during the ascent you will eventually use the deco gas you bought whereas I will use the onboard gas in the O2 tank for decompression and in most cases (admittedly not all) I'll be waiting for you to finish breathing that expensive gas while hanging on the line.
Open Circuit and Closed Circuit are two distinctly different tools. You have to decide which tool is most appropriate for you. That said there is no way a CCR costs more per dive than Open Circuit - no way - no how. My wife dives her OC Tec Rig right next to me on my CCR in Lauderdale frequently, and I cannot WAIT for her to make the switch because the OC rituals that include hauling tanks, using dive weights, and large gas bills sucks.
When I bought the O2ptima, I calculated the number of Trimix fills in a set of doubles it would take to break even - it doesn't take as long as you might think once you start enjoying helium like the DIR people do.
The question becomes, will that 100 Steel with whatever mix you choose give you the runtime I can get? If you ever start calculating the cost of a dive by cost per minute then CCR becomes quite handy. The one-hour drop given by the Captain on a 120-foot deep wreck is now 53 minutes of bottom time and 7 minute ascent for $3.00 in gas versus a fill in the blank SAC rate multipled by gas cost on Open Circuit.
Howard Packard was quoted as saying the bubbles of every Open Circuit Trimix dive he ever did were shaped like dollar signs. He STILL claims not to be able to explain it, but SWEARS it to be true.