The least expensive, aside from lucking into cheap outdated film, is probably Freestyle's "Arista.EDU Ultra" which is rebranded Foma, though I just checked and it has gone up a bit since the last I bought at $2.89 a roll. Now it's a touch over $3, $3.09 for the 100 and $3.19 for the 200 and 400. Still pretty inexpensive. I have several rolls of the 400 and a box of 4x5 in my film fridge. It's definitely old school (though thankfully I've not experienced the pinholes that some have) but kind of fun. Definitely grainy, the 400 looks like 400 film used to look.
I've never seen anything like 20 rolls for 25 dollars. If I see that I'll buy some.
I just bought 12 rolls of E100G from Thailand off eBay. No fees, duties, etc. other than the shipping from Thailand, which is pretty cheap. I've had the same experience with "Agfa" (now re-branded Fuji) Precisa film from Hong Kong. In many countries they can ship to us for prices that are a fraction of what it would cost us to ship it back to them.