shelled peas are the best cheap anti-constipation treatment for bettas, you have to mush them up. a single pea can work wonders, so a bag of frozen ones can last a betta's lifetime! iv never ever brought a bucket from a fish shop, you can get them for £1 in many cheap shops, if buying multiple tanks or a large tank: gravel can be brought in large bags as opposed to the expensive coloured stuff. i buy the largest bottle of water dechlorinating stuff to save money (although check prices as occasionally it can work out more expensive if they price it wrong). I usually buy my regular flakes from the super market, since they are the same brands, but cost less. You could buy 1 or two expensive fish and have a couple of groups of 'pocket money fish' making the tank look well stocked without blowing a huge budget (although that is obviously a personal decision).
one of the large chain stores do FREE water testing, in front of you (so you can make sure they do it right!) and this saves money if you live close enough to one, you can save your own testing kits for emergencies. (ask around i daren't mention the name in case we get in trouble!) If you are willing to nurse a fish back to health, you can get some good deals on sick fish :( but only from bad pet shops, and treatment can be expensive.
usually when i am cooking dinner I save the best of the vegetable scraps, (you can always cook them in the pan you use to cook your own veg in, if you want to part cook them) and dice them finely and pop some in the tank for the fish.