Cheapest viable disposable, without a doubt, Tranquility ATN. Reliable, absorbs 32 ounces and does a pretty damn good job keeping you dry as well as your clothing assuming it is changed before the capacity is exceeded. These can still be bought for around $1/each. Places like LLMedico that offer discounts to some people as well as rewards (works out to 2 free cases of Betterdry per year for me, on average) will bring the cost down even further.
Now, the "best bang for the buck" seems to be the Betterdry but it depends where you buy it. With the veteran discount and rewards through LLMedico, these cost me under $2/each. You can get similar absorbency with Tranquility ATN by adding a Tranquility "Super Plus" Contour Booster pad, though unless you wear Size L or larger, stick with the smaller of the 2 Tranquility contour booster pads. You'll come pretty close to similar absorbency and more thickness (if that's what you're looking for) with the ATN + booster pad and you'd be around $1.60 or so each, assuming the discounts.
I can't give you any insights to the ABDL diapers as I won't wear them. There are of course AWESOME diapers from Northshore but they all tend to be on the higher side of $2/each.
Please feel free to ask any questions. I've had to wear for 20 years now and have worn just about every medical diaper on the market, including cloth and waterproof pants.
CptKirk