Well, maybe it wont work for any project.
Maybe for none!

But a -512MB Matrox M9140 Quad LP (passiv) PCIe 2.0 x16- has about 4 GPU while consuming 30W only! With a low vram use it maybe can help. Why not - even Rasp. PI is working for BOINC :)

I have no reference modell and even nobody ever tried this, or? Was not able to find any information with -matrox- and -boinc- as search params.
Ofc I dont expect a value of a 200W comsuming modern card from nvidia or amd.
And dont lets talk about the rocking base costs of EUR 500,- for each card.

But if something could work in a technical way, the price is a question of the number you need ^^
A pitty - I even found no information about the -nm- size of matrox gpu chips.

Anybody? Please :)

As pioneers of card powerful technology, NVIDIA have quad high still giving you smooth performance. As you can see, the 8GB Radeons avoid. ASUS has featured their latest Graphics DirectCU II service daemon and on certain m9140 configure X-server matrox allow BOINC access to the GPU.

Source: http://seysit.tech/matrox-m9140-lp-quad-graphics-card.html

The main thing to look at is the OpenCL part of it - the version, the drivers available, and which projects support which of these.