I forget the site I found it on, but it wasn't totally my design.
First, buy a CO2 tank (empty) on ebay. I think I got mine for $85 on ebay. I found a gas supply place 10 minutes from me that fills/exchanges for around $25. This is where you save the most money. I fill a 20-lb tank for $25. I think the guy online said it should make 700 gallons. Yes, gallons.
Then you need a CO2 regulator. This basically makes the gas a usable pressure. Make sure to get a CO2 regulator as it has a different coupling than most gas regulators. I got one on ebay for about $25.
The next part is the most ingenious. Take a cap of the bottles you plan on using, and drill a hole through it. The guy online suggested using a tire valve with the stem removed. push that through the hole, making sure its airtight and then taking a hose attach it to the valve. Attach the other side to the regulator, and voila, a seltzer maker.
Cost:
Tank-$85
Regulator-$25
Tire valve-$3
Random hoses and attachments-$15-$20
Gas fill-$25.