$30k salary seems really low, even for entry level, but it's not uncommon to compensate for that with an agressive commission or bonus schedule. At the end of the day, a so-so sales person selling $500k a year is probably only worth about $50-60k per year, which leaves about $75-100k of gross profit for the company depending on the benefits package. IMHO, a security sales person selling systems -- not alarm contracts or near free intrusion systems -- should target $800k - $1m per year in sales. Smaller markets may be less, but unless they can bring in at least $600k, then it probably isn't worth it to either of you.
Here's a thought based on previous experience, you might want to consider paying based on the margin amount rather than the gross. This has become common because it puts some skin in the game for the rep when they see that selling at 40% puts more money in their pocket than 33%. Those margin points really add up and its good practice for sales reps to understand the benefit of improving margin. Back in the day when I was paid this way, I did everything I could to make sure that each job was as profitable as possible, so I know it works.
You can also consider a scaled approach where they earn a lower percentage on sales up to $500k, then a larger amount for sales up to $750k, $1m, and so on. It takes some thought as to how you would organize this and manage it, but it pays for performance, which is ultimately what any desirable sales person would prefer. If they're content with a higher salary and lower commission, you should know right away that they're not the best fit, at least as a sales person.
Just my 2 cents...good luck!