Depends on what you mean by "fix up" and what you want to end up with. The bike looks pretty much ready to go. Basic servicing and maybe tires and brake pads. If you D-I-Y the cost will be minimal. Add a rack and some lights and you are on the road pretty cheap. I commuted on a '77 Schwinn Le Tour II for quite a while. It is similar to the Miata you linked and it was a solid commuter. Not light but neither am I.