ok it depends if you're talking for track/autox use or for use in a road car.
for track/autox the cheap racing seats like Corbeua are adequate. Not much more than that. Those seats meet only the lowest standards. A real racing seat is ultra expensive because it meets much higher saftety standards. I think
Cobra make some very good Kevlar seats for the money.
For road use I would never use a budget Momo, Sparco or Corbeau seat. You are way safer in your Porsche OEM seat which has at least gone through extensive crash testing both here and in Germany.
A while back I ordered the Corbeau A4? My first mistake was buying a racing seat that reclines. That's your first red flag (fake). When the seats arrived the materials where crap. Thin fabric, low quality all around. Then I was hit with a $100 restocking fee. The GT3 seats may be expensive but they are legit.
I'm not sure I would trust any seat that cost less than $800-$1000. I just don't think its feasible to make a safe seat on a budget.