Cheapest NVIDIA GT710 I could find today on eBay USA is $35 with free shipping within USA, no affiliation, I've found it cheaper some weeks ago.
ITEM : Zotac Geforce GT 710 Zone Edition 2GB 64Bit DDR3 Low Profile video card. This card is used, pulled from a working computer and been fully tested. These fees are non-negotiable and are calculated at checkout.
www.ebay.com
It's the weakest and cheapest METAL supported GPU, but works good if you have one HD display and don't run anything 3D. AFAIK, this card can't be flashed for Mac EFI for pre-boot support.
A better option still affordable at ~$57 could be a GT 630 (one very commonly used with corporate HP PCs, the GT630 model with part numbers 684455-002 702084-001 B4J92AT can be flashed with a Mac EFI and the Mac EFI is public):
This ROM will give you EFI boot screens on the Classic Mac Pros, but it isn’t recognised by the Mojave installer as a metal card. It does give you the option of a cheap general use metal EFI compatible card, it will drive 1 monior to 4k resolutions. You’ll need a windows PC and NVFlash […]
www.loremipsum.info
The Mac EFI above works perfectly for the listed HP GT 630 GPUs, I bought two cards back in the last AliExpress Black Friday and I already flashed and installed one in my father's Mac Pro running Big Sur that have a 29" LG 29UM68-P ultra-wide display.
This is the AliExpress seller that I bought the two HP GT630, no affiliation:
Smarter Shopping, Better Living! Aliexpress.com
www.aliexpress.com
The GT630 is the "modern" equivalent of Apple's GT120 of early-2009 Mac Pros, it's single-slot and only powered by the PCIe slot. Even the Display Port audio works with Windows, didn't test it with macOS. It's fast enough for office work/browsing/media consumption and it's fully supported from Mountain Lion to Big Sur (Apple removed NVIDIA drivers from Monterey). You can't get anything cheaper than this HP card that can be flashed with a public Mac EFI.