I remember going to, *gulp* WalMart once and seeing 6 gallon set ups around $35. Don't kill me for using the store's name. Other wise, I'm sure places like http/drsfostersmith.com, http/www.petsmart.com, or http/www.petsco.com will have six gallons way cheaper.
What a handsome marble boy! He's really gorgeous and he looks super flarey welcome!!
Those flakes look fine... the top 3 ingredients of any food usually tells you a lot. Your food is made up mostly of Red grubs (blood worms), krill meal, fish meal, ALL meat proteins which is great. There should be no problem feeding him bloodworms as an only meal, however. Faith from BettaTalk, who is pretty much the most successful Betta keeper/hobbyist ever, feeds her Bettas only frozen Bloodworms and Brine Shrimp. And her Bettas are some of the top show winners and breeders in the country. Feeding flakes and pellets is what actually causes constipation, because of the fillers and dryness of the food. All of my Bettas flat-out refuse any kind of flake, and some won't even eat pellets so I mostly feed frozen Bloodworms and Brine Shrimp... no constipation or illnesses at all. Your boy looks VERY healthy to me! It's awesome that you're fixing him up with a 6g, he deserves it