okay we are talking about a 6 gallon tank
That honestly doesn't help anything - if anything small tanks are even more susceptible to the dangers of inconsistent CO2 delivery.. and you have to watch the bubble count constantly to make sure it's stable. Less than 3.5oz of CO2 will honestly not last super long - by the time you've bought an extra CO2 cartridge or two you may as well have bought a paintball tank (where I get mine, your first fill is free).
If you are adamant about the price point,
plus a $16
. Then get and fill a paintball tank (mine were about $20 each at a local sporting goods store) - it will last longer between fills and refilling one is usually $3-4.
Now, it's probably not a solution many people would really recommend (it's basically the same $39 kit, but not in a kit), but it would avoid the money pit that the disposable systems are without making you spend a lot more now (only extra is on the tank, but you would have spent that within a month using disposable cartridges), and it would set you up better to upgrade to something nicer later.