If you have an MSDN (not MSDNAA or DreamSpark) subscription, then you get the level of support provided by MSDN, it will likely vary so I won’t quote the monthly amount available. (Ok, it’s $150 per month provided to you for no cost on 6/12/2014.)
So here is the cost for messages, and we will be sending messages to the Service bus from the Netduino or Arduino, here is the cost: $0.01 per 10,000 messages. This might seem like a lot, but let’s say you are building an aquarium system and plan on selling it broadly. If you are successful, and why not, then an aquarium household might send out 200 messages a day, let’s say. If you have 50 units sold, then that is 10,000 messages per day. At the end of the month that might be $0.30 or $0.31 per 50 units. 500 units $3.00, and so forth. So you do have to budget for these costs. And no matter which system you use, Microsoft Cloud/Azure, AWS, IBM, or others you will need to budget for this cost.
For more information on how much the Service Bus costs see:
If you are a student, if you read over that page it will help in interviews if any questions come up in the interview about how to cost out the cloud. Why would you be ask about the cost of the cloud usage? This is usually a test to see if you understand the real world issues of costs and so forth. No cloud services are going to be free, someone has to pay for the lights and air conditioning.