Good catch! I forgot to add in for two tubs of Oxi. So now, the actual cost of homemade PBW per pound is more like $3.86/lb instead of $2.77/lb. That's still pretty good, imo.
Yeah, if you've got a Costco membership you can order the OXO brite online and bring your cost way down.
My new plan is to use the OXO brite from Costco + the TSP substitute you found from Amazon + the 7th Generation from Amazon/soap.com.
That should bring my cost down to about $2.20/lb.