Assuming you immediately switch to the larger 88g cartridges, you're looking at 27 cartridges, or 9 3-packs, to equal a 5lb cylinder.
Granted, your tank is quite small. But, you wanted total cost, so here's a very general breakdown.
A5lb cylinder might last 12-18 months on that 9g. In those 12-18 months, you will spend:
Cheapo reg, including solenoid valve: $100
5lb cylinder: $65
First fill: $20
Odds and ends: $25
Total $210 (Add another $150 for a dependable rig with high quality components, so about $360)
Fluval mini CO2: $30
9 3-packs of 88g cartridges: $324
Odds and ends: $20
Total: $394 (Multiply cartridge cost by 3 since there's no solenoid valve, and CO2 will be on 24 instead of 8 hours a day, so about $1022)