You could wear them out in 2 years, maybe 1 year, using them to store off-peak power for use on-peak. $0.29 - $0.11 = $0.18 per kWh saved. Without knowing purchase price or rated cycle life, I'll assume the batteries cost you $0.50 per kWh, so you lose $0.32 per kWh this way. Not counting cost of inverter. Will that automatically switch back to grid when it reaches low battery shutdown?
The only thing this does for you is delay when you buy a lithium battery. Then you get to address precharge (is battery willing to be connected to an inverter, or does it shut down? If necessary, use resistor to charge capacitors.) Address peak draw (when inverter starts motor in laundry machines, for an instant it draws 5x running current. Lithium battery and BMS must be sized large enough to support that.)
Lithium batteries may be cheaper still in the future. Or may be more expensive. I think the price is based on volume of reject batteries not good enough for EVs in China, and market demand for them.
AGM batteries are simple, robust, easy to use, but need to be fully and properly charged, including a couple hours of absorption. Best use for them is portable applications where their weight is not excessive (permissible as air cargo) and backup applications where they see a few hundred cycles over their 10 year float life.
I suggest using them as backup for critical loads. My internet gear has 12V plugs (I assume but don't know that they would be fine with the range of charge and discharge voltages used or AGM.) Possibly your computer would too, or you could get a small inverter for it.