https://bundleofholding.com/presents/ArdenVul
The Halls of Arden Vul is up on the Bundle of Holding (2 days left). Not sure if this is the largest megadungeon on the market, but it's good and you're getting 1100+ pages of adventure and lots of art and maps for a mere $24.95 (the regular price is $109).
See a review at
https://tenfootpole.org/ironspike/?p=6633