Worried about messing up my first whole sale Deal. Help!

I will try to make this short, 

I have a home owner who is selling his home on a 1 acre lot/parcel for about $30,000 less than the fair market value. I'm estimating about $5,000-$7,000 in repairs/improvements. I was considering trying to make $7,000-$10,000 for my wholesale/marketing fee. the property is owned out right, and I'mestimating closing cost to be around $7,000. I just terrifiied of messing my first deal up by over charging my investor. I think paying $20,000 below fair market would be a good deal for an investor?