Thank you, everyone, for the 101 course on what those weapons do...
Technology costs are calculated by tech tier, they don't factor in stuff like how many different sizes the same component they unlock comes in.
But techs at the same Tier aren't all the same cost (e.g., Tier-2 techs usually cost either 4K, 5K, or 6K). The OP was asking if the techs that provide less benefit, in terms of fewer slot options provided, should cost less than the techs that provide more.
For instance, and tying back to
this post I made a couple of weeks ago, a Kinetic Battery (Tier-3 but only L-slot) is 12K tech past an Advanced Railgun while a Gauss Cannon (Tier-4 and S/M/L) is 16K tech past ARG. The KB and the L-slot GC have the same Alloy and Motes costs (although more Power is needed for the KB) and end up netting out at the same effective damage (against a balanced Hyper Shield/Neutronium Armor Battleship) and cumulative disengage chance (against a Cruiser). Even if an empire eventually researches both KB and GC (if they plan on researching Kinetic Artillery, they have to), and then would never use an L-slot GC, there are still more benefits if an empire only has GC versus only having KB, justifying IMO the higher cost.
As I said in the OP, and especially if the thought process is "L-slot Plasma isn't going to get used
once you have Proton/Neutron Launchers", should Plasma Launchers even have an L-slot option in the first place, even if there were a higher tech cost relative to Disruptors or Autocannons? If they only cover S- and M-slots, should their effectiveness be more like an "advanced" weapon (like PL/NL, that have outsized effectiveness relative to Tier) versus a "specialized" weapon (that does its schtick (e.g., anti-Armor) well, but its weakness holds it back overall)? If so, I'd probably recommend upping their tech requirements to include the same Tier basic weapon (e.g., KA requires both KB and GC, so Plasma Cannons would require X-Ray Lasers and Plasma Accelerators would require Gamma Lasers).
As a bonus, letting S- and M-slots get some of the KB/KA and PL/NL kind of love would perhaps serve as a way to make Destroyers and Cruisers more viable later on.