A CO2 system can range in price. However, the average cost comes down to around 200 bucks, with an additional $5 monthly to refill CO2. You can go for more quality items that will up the price to $400 or cheaper ones that might cut it to half. That all depends on your budget and how much you're willing to invest.  However, if you find it hard to invest in a CO2 system right now, maybe hold off on getting that planted tank until you set some money aside.