Last I checked the price point, return on investment, was with conventional flooded lead-acid cells for battery chemistry. Yes, gel-cells, AGM batteries, have a lot of advantages, most of which don't make a lot of difference in small PV systems type use. Flooded lead-acid batteries are old school and very rugged. The big thing to remember with a FLA battery is that for all sorts of engineering reasons the total practical output is one third the total capacity. Limit it to that, with only the occasional excursion, and you multiply the lifespan.

In these the best form factor was in what are termed 'golf cart' batteries. These get sold a lot and account for the majority of the market so there is a pretty lively competition keeping the price low while, at the same time, making them the easily batteries to find.

AGM batteries are coming up fast in use, and have been for twenty years, and they might be the way to go in some situations but for a stationary system FLA still gets you more for your money.