One of the biggest mistakes I see do it yourselfers do is cut corners without even realizing it. Pride of doing it yourself and getting it done seems to allow the person to overlook things that they would never accept when hiring a pro. When I build something for a client, I size materials to code or larger, even when there is no code. Since I'm doing it for hire, it has to be right and it has to last. My reputation and future depends on this so that I can make a living. I have tens of thousands of dollars in tools that most home owners don't have, and don't even know exist. Just like hiring a mechanic, you are paying for everything that he needs to get the job done, but also his experience. Not knowing, or even caring what you make as a mechanic, I would never insult you by saying all mechanics are crooks because they charge so much to fix something that I don't know how to fix, or I don't have to tools to do it, or I just don't want to do it. I would like to think that like a mechanic, my time is worth something when somebody hires me. Nobody has to hire me, and pay what I'm asking. That's on me and every other builder out there to come up with a competitive price that will get the job done, and done right. Low balling the price and cutting corners is never an option, so you either pay what I'm asking, or find somebody else.
As for building it yourself, this is about as easy a build as there is. Materials will be the big deciding factor in what it costs you. Go cheap with 4x4 posts and hope they don't twist on you after you are done is a risk that will save you money, but not one I would take. 6x6 posts will cost a lot more, but they are what I'd use. You need to decide your spacing of posts, then based on that spacing, size your purlins to the span. Again, it's easy to cut corners by using small lumber and maximum spacing, but then you get walls that more around and come loose over time. Nails are fast and easy, but they work loose over time. I never build an outdoor structure, or deck with nails. The roof is where you have a lot of options. A free span, flat roof on an angle is going to be the cheapest and easiest. I've seen people do this with 2x6's and get away with it, and I've seen them sag with 2x6's. I would use 2x8's on 24 inch centers with blocking down the middle at a minimum if you don't get snow. If you get snow, that needs to be factored into the sizing of your lumber and roof design. Building a gable roof with a pitch steep enough to handle snow might be what you need, which will increase the cost of the roof by at least twice, probably more. Shingles or metal for the roof? This also affects the cost since each go on differently and you need to plan for that. Shingles attach to sheething and metal attaches to purlins.
My last point is that most home owners do not know what a good contractor is or how to spot one. They often hire friends or family because they know that person and assume that he is going to do a great job because "everyone" says he is the best. These are the jobs I get hired to redo the most often. If somebody is good at what they are doing, they are not available to do it right away. They will be booked up into the future and you will have to wait for them to start. Right now, I'm booked up to October. Angies List is a good source for finding a contractor.
Eddie