Testing a cold-audience ad on a $400 budget to see if I can generate proof

I run sponsorship placements for a few clients but I have nothing concrete to show a new buyer who asks what a cold audience actually does with one. So I put $400 into a single sponsored slot in a niche newsletter outside my usual circle, same creative I use for clients, one clear offer. It's small and it might tell me nothing. But it's the first time I've paid to find out instead of guessing.