This is actually a very good question with complex answers.
Price per watt. If you buy anything in a large volumes it will obviously be less expensive than if you buy it in much lower volumes. Currently the price per watt for utility-scale PV (in the US) is about $1.10. For residential-scale it's about $1.60. (Source: NREL).
Taking advantage of multiple value streams. Battery energy storage systems (BESS) can provide multiple grid services. However PJM was a pioneer in implementing these in their grid, but only focused on frequency regulation. As a result the BESS implemented on their grid are configured to only provide this service. California (CAISO, CPUC, etc.) came in a few years later and learned from this and made sure their rules allowed the BESS to support multiple services.
Utility-owned dispatchable generation (BESS or PV+storage) can probably be repurposed to focus the dispatch on a particular function or service fairly easily. Distributed energy resources use a tariff and the conditions therein to define how the owner (facility or residence owner) dispatches them, which is generally in a manner that results in the best financial outcome for the owner. Changing an existing tariff is not a trivial matter.
There are other considerations. It just so happens I will post the first part of a two-part series (a long paper) that delves into this issue rather deeply in about a week, and the second part about a week later. Check into my public site.