I just made this schedule.
The time period of it is 4 weeks, devided into two 2-week periods.
The picture above the period is a situation at the beginning of those two weeks.
2W and 0W stands for 2 and 0 weeks already at the position (TAP, SECONDARY or PRIMARY) respectively.
The number in the keg/kettle stands for a batch number.
The numbers below the line are specific numbers of my kegs, so that you can see how they travel between the positions in time.
Everything OK? I like it because I get 2 weeks primary and 4 weeks secondary so the guys are not so green. In case of a beer stuck or too slow at tap, I can rack a batch or two from a primary into my carboy, so I get a batch or two as a backup for those faster drinking months.
I also like the idea of one full and one half empty keg on my tap, so I have one change on tap every two weeks instead of two changes every four weeks.
I want to start with just one gas tank with one regulator, so priming sugar till I get one more.
Way too detailed for me.
But, I think I can make some suggestions:
1) Don't worry so much about getting things out of a primary. Let it sit there a while (as long as you have a place to keep it moderately cool, e.g. a basement)
2) No need to secondary, unless it is something like a fruit beer.
3) You can keep stuff in a keg under Co2 without putting it under refrigeration. Again, as long as you have a moderately cool place to keep it. This leads into the "you can never have too many kegs discussion".
4) When I get tired of something, or can't wait any longer to get a new batch to a keg (cause I want to drink it), I bottle from the keg. This works great:
https://www.homebrewtalk.com/f35/we-no-need-no-stinking-beer-gun-24678/. This gives you the best of both worlds...bottles when you need them/want them or just to free up the keg.
5)There is no need to carbonate with Co2. While I have 3 Co2 canisters and regulators, I've found that I only use one at a time. I don't leave my kegs in the keezer with the Co2 attached. I've had slow leaks and one time lost 20# of Co2 (not nice). So, I just carb them up by attaching Co2 (first at 25#, then at closer to serving pressure) and detaching the Co2. Works great, and you can always attach the Co2 if you are having a party or something.
I know you said you want to keep the cost down, but think of the kegs as:
1) The place to carbonate and drink from
2) The place to condition
3) The place to cold crash
4) The place to keep a ready to go pressurized Star San.
5) The place to keep soda (cheap and very easy)
6) You can even use them as a primary (I haven't done this, see threads on this:
https://byo.com/stories/issue/item/1763-ferment-in-a-cornelius-keg-projects)
As a type this, I have six kegs in a refrigerator, a keg with SNPA (clone) in the basement (conditioning/carbonating), and a keg with birch beer in the basement (carbonating). Once a day or so I go down and check the pressures and add Co2 as required.