Building a 30 x 30 garage yourself costs between $7,000.00 to $12,000.00 depending on the cost of material in your area and how nice you want the garage.
That cost also includes laying the rebar and pouring the concrete slab which is not that expensive but still not very cheap and you also need to level out the dirt where you plan to build the 30 x 30 garage so that's another factor and whether or not you want metal roofing, siding etc on the garage.
But to build a 30 x 30 garage I would budget at least $10,000.00 but it could be a bit higher in cost.
Sometimes it's just as cheap to just hire a garage builder who can come out and build your garage for you in just one or 3 days.