I'm in north Texas and work for a relatively small HVAC and plumbing design/build/service contractor. If we do design work, we normally bundle it into the overall job cost to secure the installation. We actually haven't ever done purely design work (providing stamped engineering drawings for someone else to install) because we don't have insurance for that. However, I believe the going rate around here for a small design firm is about $1.50 per square foot per discipline. For example, if you design only HVAC on 10,000ft^2 building, design fees for providing stamped engineered drawings would be $15,000. If you designed both HVAC and plumbing for 10,000ft^2 building, fee would be $30,000. That will at least get you in the ballpark.