Not the least expensive but my preference for kits with a single action trigger. Watch to see if they have any BF deals.
https://algdefense.com/ar15-m4-mil-spec-lower-parts-kit-less-trigger-less-grip.html or with the ACT trigger (very good single action) https://algdefense.com/complete-lower-parts-kit-with-act-trigger.html
https://algdefense.com/complete-lower-parts-kit-with-qms-trigger-595.html
Last year on black Friday Stag ran a sale on the LPK without a trigger. $14 each. Pair that with a Larue trigger and you've got a good setup for little money.
This is the kit they offered. I bought 3 of them, have 1 left, will be getting a few more if they do it again.
https://www.stagarms.com/lower-receiver-parts-kit-without-trigger-group/
...You can buy ALG triggers pretty cheap if you can wait for few months till the NRA meeting this year in Indy. They had ACT triggers at $50, QMS at $35 last time few years ago.