Well there are 2 scenarios that I am aware of.
Large companies charge support A LOT meaning that 1 annual contract allow them to cover yearly costs of the project. But those are large companies.
I personally charge my regular work hours + 50%. For example, a client and myself reach a retainer agreement where I promise that I will reserve X work hours just for him in one year and any bug which appears, I will fix asap (a day or two if not immediately). If the client finds it expensive, then I ask him to contact me in case of problems but I will fix them when I have time (and charge my regular work hours).
Now, regardless of the fact you're subcontracting, you subcontractor has a job because of your client and your reputation is on the line, not his. That is why, it's absolutely fair to charge "his rate x 2" i.e. you earn the same as himself. You will soon realize that there is no such thing as "not working anything" when you're subcontracting. I found myself managing subcontracted project 8 hours a day. Just like I am coding it.
Now, the next point is sustainability of your business. You should charge as much as you can because you must keep the business going. It does not matter is you're charging 3 or 10 times more because there will be many situations where you will not be able to charge your client, but will have to pay subcontractor from your own pocket just to preserve your reputation.