Yes, it is just much cheaper to build them than it is to buy them. Looting them is of course cheaper yet.Crafting plasma rifles might be wasteful unless you want to give all your girls a plasma rifle for roleplay reasons. You can loot them from bounties. By the time your workshop is set up to craft it, you would have a decent number of plasma rifles already. Crafting their energy cells for your own use can work though. I have not checked whether it is cheaper to just buy it or craft it yourself however.
Also, plasma rifles doesn't have any recoil so any npc can use them, which may be useful if you just want to kill one badie without hiring a gang of mercenaries to do it, or to protect your group during anarchy, when passing thru Kaseys park.