The options are to wait for prices to come down, keep checking the official resellers or the TE mktplace if someone is selling. For just pihole even the pi zero or older Pi's are more than sufficient. Other alternatives -
1. Look at other cheaper SBCs which can run DietPi.
2. You can also repurpose an old Android phone to run Pihole, using this like this person did -
Pi-hole for Android (Pi Deploy) utilizes a fork of Linux Deploy to simplify deployment of Raspbian to most rooted Android devices. Note to users of previous builds: Uninstall any previous versions...
if you have a old pc or cheap mini pc like Dell optiplex you can run pihole on it and you also have a lot of headroom to run other services you might like,
i have a home server on i3-6100 32gb ram and 3x4tb wd red running pihole+unbound in a lxc and dozen of docker containers and NFS and samba share.
This post is so cool!
I think I'm gonna try it at my parents' place, where deploying a small computer that'll run 24x7 raises eyebrows.
Old Android phones are a very good Pi replacements, I ran a Telegram bot off of my decade old Redmi phone, and it was able to service 4000+ users a day.