I currently only carry G&L insurance. We are now looking into Professional Liability. It is my understanding that since we also do our own "design" this is a must, because our G&L doesn't cover this exposure. The policy is quite expensive ($10,000) because not many companies write insurance for companies who design, fabricate and install. The $10,000 is a minimum entry cost. Does anyone know a cheaper route?

Hard to say as this will vary from state to state but I'd check with a lawyer and see if paying an outside PE to approve your design would cover your assets. Depending on the volume and complexity of the designs it may be more cost effective to pony up the $10k.

Good luck in your quest.