If you're not limited to disposables (by privacy concerns, laundry facilities, or personal taste), you could always look into cloth diapers. The upfront cost for the diapers and plastic pants is higher than disposables, but in the long run, I think they're the most economical option around.
I recently purchased a Leak Master all in one adult cloth diaper like this:
http://www.adultclothdiaper.com/PRODUCTS_2/CLOTH-DIAPERS_2/Leak-Master-Adult-All-In-One.html
Unfortunately for me, I am limited in my ability to use it since I share laundry facilities with someone who doesn't know about my ABDL side, but in my limited experience, there's nothing more satisfying and "real" to my AB side than wearing that cloth diaper under an adult onesie with a paci in my mouth! Super regressive!
There are other options for cloth, too, which can get way more economical than a purpose-built all-in-one style diaper. I just chose the all-in-one style because I'm not a huge fan of plastic pants, and the particular all-in-one I chose comes with a waterproof PUL outer cover (the same material as high-quality plastic pants).
One downside, in comparison to ABDL-oriented disposables, is the lack of cute prints available, but I think there are plenty of places to get very babyish onesies that will amp up the "little" factor immensely!
tl;dr Cloth diapers are a cheap option and there're good ways to make them babyish.