i think maybe she means 17L for 20 dollars? maybe....thats what i have, a 17 litre that cost 35 pounds as a kit..gravel and filter included
The pet store I work at (very well known, so won't mention names, but it's one of the "big ones" in the US) sells 10 gal tanks for the same price as 5 gal (no I don't get it either.) The price of either? $13.99. Get a bag or two of gravel for $3.99 apiece (your choice of color!). Filter for $10. Heater for $10. And a little place for him to hide for $2-$3. That's, as I understand it, bare minimum for a happy betta.
Total cost: under $50. It's more than that little supposed "all-included" cup, but your betta will also be happier and healthier and live a longer life. Plus, you get to decorate it yourself!
I know all these prices off the top of my head because I am planning on getting a betta soon for myself, so I have looked into how I can give my little guy a good home, while not breaking the bank

(That's why I'm on this forum, looking for all the information I can before I take the plunge, to use the phrase. Hi!)
Oh also check out Freecycle.org. I don't know if they've got a group where you live, but this place is great for getting and unloading free stuff if you do. I almost got a tank through them, but the guy flaked at the last minute and never gave me an address to pick it up (drat... would have been a free 20gal!)