There are a LOT of factors to consider when determining a rate to charge a customer.
You need to charge enough to make a profit without charging so much you do not get repeat business.
If this is full time self employment, you have to cover your overhead (taxes, insurances, expenses) office?, vehicle, fuel, tools of the trade, code books, CWI fees, safety items harness, PPE's, etc.

You also have to determine what you need to make ends meet. You don't mention if you are already in business, going in business, work for a company and expanding so more info would help with better answers.

Your geographical location will also be a huge factor. What competition is in your area and what do they charge? Who are the customers? Your rate may vary if you are testing an unemployed welder that needs/wants to get certified or you are testing welders for an oil company.

You want to get business but don't charge so little that you undercut the competition. It will hurt you in the long run. Plus, if you charge half of what your competition charges your prospective customers will think you are CHEAP. 

Supply and demand will ultimately determine the rate that you can charge.

Ahhh...capitalism. A wonderful thing.

I asked more questions than I answered but a ballpark figure would be 55-80/hr for general testing. As mentioned earlier...a lot of factors.

Good luck with your venture.