Building a bike is much more expensive than buying a complete bike... if you currently do not have close to enough parts to piece together a bike. Your best bet is to look heavily on the used market for a complete bike. Look for used surlys and salsas.... a lot of them can be found with rigid forks.
Your position of entering the world of biking and trying to figure out a way to do it cheaper is a common one. I was there for sure. The problem is it's hard to realize how many parts you need to build a bike until you actually make a spreadsheet. Parts like.... cranks, headset, bb, axles, tires, blah blah really blow the budget out of the window. A good way to go is to find a good used single speed for sale that has the potential to be upgraded down the line if you wish. That way you don't have to pay for all of the drivetrain bits right off the bat.
Your position of entering the world of biking and trying to figure out a way to do it cheaper is a common one. I was there for sure. The problem is it's hard to realize how many parts you need to build a bike until you actually make a spreadsheet. Parts like.... cranks, headset, bb, axles, tires, blah blah really blow the budget out of the window. A good way to go is to find a good used single speed for sale that has the potential to be upgraded down the line if you wish. That way you don't have to pay for all of the drivetrain bits right off the bat.