They're more expensive. All I've seen around actually for sale are small ones at lighting places but seemed to be around twice the price, then there are aquatic ones that you'd have to look hard to find around here and they're even MORE expensive.
Calcium based, actually.
I estimated the cost for me with my smaller tank to be around $100/ month and that's only with small water changes on 200L tank. So, IMO better moneywise collecting seawater.
You can also DIY one, mine cost me about $100 including a heatgun to bend some perspex. Still needs some refinements, but just an air pump, jerry can, float valve and a few other little bits.