As I understand it, your goal is to reduce your expenses. That immediately means don't replace anything you now have, unless you can sell what you have for more than you will pay for what you would replace it with.
Your monthly plant related expenses include electricity, CO2, fertilizers. Your light is way too much for a low light tank. If you can sell it for a good price you could save a little there. Or just run two widely spaced bulbs, put a layer of fiberglass window screen over the light (cuts the PAR by 40%) and have it at about 24 inches from the substrate.
Since you already have the CO2 system I would keep it, and reduce the bubble rate so you have around 20-30 ppm of CO2 in the water. That should cut down on your CO2 refill costs. You can then reduce your fertilizing to about half or less of the EI dosages, slightly reducing that cost.
None of those cost reductions will amount to much, but you should then have to do far less tank maintenance, spend a lot less fighting algae, and save a little more that way. I think I would just sell this change on the reduced work involved, the more relaxed approach to tank maintenance, and therefore more time to wine and dine your wife (killing all of the cost savings, but greatly reducing the wrath.)