RODI + salt mix is ideal. There are some sites out there that explain RODI systems pretty well, plus i'm sure a bunch of us can chime in with tips if you need help starting up with a system. Bulk Reef Supply has a decent video series that can give some starter info. $10 for 5 gallons is rather costly over time. RODI + salt has an up front cost, but over time it can be cheaper.
You can prob find a decent deal on a used RODI unit. But count in the cost for a new membrane (~$40) and filters+DI (~$40). A used unit may have old filters in it. Membranes generally last 3 years. Filters are typically changed yearly. DI units usually when filters are changed. The rate of usage of course can vary depending on your water quality. In my case, my water at my home is very clean. I am at 15 months on my same filter set, but I will change them out soon. Water quality can be verified with a TDS (total dissolved solids) meter. A rather low cost device. Some RODI units have them integrated.
I change about 5 gallons weekly on my 30 gallon+sump system. If I used petco water, that would be $20+ a month, or $240+ a year. At that price, one can get a quality RODI unit plus a 200 gallon box of salt-mix. You can then make RODI water for topoff to replace evaporation, and make water change water.
I typically keep 2 5 gallon buckets of water going. One bucket is pure RODI. The other is mixed salt water. The pure RODI bucket is used for topoff water. The other for weekly salt water change.
Remember... try to mix RODI and salt mix ahead of time. I try to let it mix for 24 hours at least before using it for a water change. It gives that water time to mix well, settle, and aerate. Undissolved salt mix is rather caustic/nasty, and your tank critters won't be too happy.
