There are ways to limit the waste as well. I you added another carbon filter and membrane.....although you may go through a membrane quicker on the second unit, it then reduces the waste water and yields you twice the treated water. For fish hobbyists this may be a good option unless you go through 100 gallaons a day of use.
And keep in mind it may appear all are near the same, that isn;t necessairly true. I do not think you need to buy an expensive canister, but as Angela had done (and I did as well), check the fittings and ensure theya re good fittings.
The real difference are the membrane and particle size ratings. You have a choice here. Most less expensive units come with 5 micron filters, and a non TFC membrane (I forget the other type of membrane).
TFC membranes do a better job and last longer imo. Also, 5 micro filters do OK, but 1 micron do better. If you have very little TDS to begin with and your water is soft natively from the municipal supply, then the cheap filter elements may work out ok. Otherwise, 1 micron is your best bet. Don't forget to backflush them every 1000 gallons or so.
Another way to hep save money are using some refillable canister inserts for your DI or carbon. BRS has them and the DI refillable resin pretty inexpensively.
Regards,
Andy