"Impact" is right. Everyone on here is talking about different things. I work for a $50 million+ contractor and have worked for larger ones and I have noticed that often times the big difference between the large contractors and small ones who want to grow but are struggling is that the smaller ones struggle to identify all costs. You want to focus on what you want to can achieve for a gross profit on the project.
Gross Profit (in my mind and I was an accounting major before I got into this crazy business) should be defined as:
Gross Profit = Total Revenue - Cost of Direct Services Sold.
Cost of Direct Services sold would include your subs costs, any time you spend on the project (if you will be there 10% of the time you will need to take 10% of your expected salary that year and include it), any equipment support you will have to provide and any materials you will have to expend.
It does not include your home office Overhead/indirect costs (accounting, rent, utilities, secretaries etc).Your home office overhead should not be considered because it is a fixed cost. At the end of the year it will not change. Only the amount of revenue you brought in will (unless you hire more management etc.)
Like you, we self perform some work and sub out specialty work. What I would suggest is doing this exercise in reverse, figure out how much your work will cost and how much the subs are willing to work for and then combining them and you will have all your costs for the project first. Then look at your costs and determine what you want your markup to be. For us, as a large contractor doing major projects in the Boston area it is usually 10-15%. This is a highly competitive marketplace and don't forget that we also have to cover our home office overhead. At the end of the year we usually end up with somewhere around 8-10% total net income on work (implying we have 2-5% home office overhead). You do smaller work and possibly the markups are larger. However, if the markups after ALL your costs are 30% please tell me where you are working because I need to come start bidding jobs. No contractor regularly makes 30% after ALL costs in a competitive marketplace because there is always someone willing to do it for 15%.
As for issues you should worry about I would start with getting quality subs you trust. I cannot stress the importance of this. Incompetent subs just bleed money for a GC. If there is a guy who is slightly higher in price but will do the work properly with nearly no supervision...HIRE HIM! Secondly, you need to get a strong contract to lock these guys up. You should be able to find one online for a smaller work but if you get into bigger projects hire an attorney. You do not want them being able to walk off jobs for no reason with no fear. Thirdly, I would suggest holding retainage on him until you are certain the work is complete to a high degree. 5-10% should work and usually will allow you to back charge him if you end up having to complete something he didn't do or did poorly. Release it only once you are sure he has completed everything properly. Fourth, the people who said insurance were very right. Make sure he is licensed and insured (put it in the contract). Lastly, I would remember that if you are a GC it is now your reputation on the line for other peoples work. You need to remember to be tough with them and be demanding of quality, safety and respect to the customer.
Boy that unintentionally got longwinded. Good luck!
. If you don't get a yearly insurance audit, this may not apply. 
