This is kind of an open ended question and has a lot of variables. Depends on what kind of diving you’re doing, what unit you are looking at buying, do you plan on furthering your diving carrier (going deeper, longer dives, maybe cave, wreck, overseas travel etc.) Also it’s hard to justify a rebreather only off gas savings. I think most of us that dive CCR's have more reasons than just the gas savings, yes it’s a big bonus but there are countless other reasons for diving a CCR.
First thing to remember is the initial cost, this can very a lot depending on the unit and if you buy new or used. Also next is the training CCR courses generally are a lot more than OC courses.
Once you own the unit they are actually very inexpensive to own assuming you don’t brake anything on it. Once again this is all depends on the unit you buy but lets just keep it simple. Most manufactures have you change O2 sensors and replace ADV, DSV, and T-piece o-rings. Every year and every 1-2 years replace all other o-rings. I would say just ball park you will be spending $200-$350 a year. This is unit dependent, some units have cheaper aftermarket o2 cells you keep the cost down, other you must use manufactures cells and some have CO2 cell as well. Also most manufactures recommend on average every 5 years (this number is very unit dependent) to send the unit in for a manufacture service. For now will just leave this out since it’s such a long period of time.
Also as usually most manufactures recommend servicing your regs annual as well.
Now if you are just doing this to save gas cost then all you need to do is find out how much your local dive center will charge you for a diluent and o2 fill (depends on the tank size you go with most people dive 3L’s) now times this by your 20 dives (in reality you should get a couple dives off one dill and o2 fill) a year and compare that to your 20 OC dives a year. Then subtract the maintenance cost and sofnolime cost and you have your savings per year. You will really start seeing a difference when you are doing deep dives with very high % He diluents vs OC twinsets and travel gases.
But the catch is you still have to divide the annual gas cost savings by your unit cost and training cost then this will tell you how many years you will have to dive be for you break even between OC vs. CC obviously the more you dive the quicker you reach this point and that is when you gas savings start kicking in meaning that is when CC become cheaper than OC
Also remember if you travel a lot like I do have to do a little more research on dive destinations to make sure they can support CCR diving, general remote areas charge a lot for sofnolime and He, also I have found a lot of remote places can not fill 100% O2 I get a lot of 93%-98% fills. You will also be traveling with 2 full checked bags (assuming you have a full size unit not one of those little around the neck lunch box Tritons) and nowadays most airlines only let you have 1 checked bag and pay for the other and by pay for the other I really mean bend you over can you feed you the pineapple for the other bag, especially in Europe my friend gets charged 150 Euro for a second checked bag on Lufthansa. I am lucky and I fly so much that I am United 1K so I never have to pay for additional baggage on any Star Alliance flight around the world. But I have friends that spend around $400-$600 a year in extra baggage charges. This will actually be your largest cost of ownership is travel. On a side note myself and lots of my friends fly Turkish Airlines and if you have scuba diving gear they allow you 2 free checking bags, and if they give you a hard time about it pull it up on their website.