I understand about the money thing. I would see how things go once you install the new plug. I had a faulty relay in my '77 300D one time that showed the same symptoms. The plugs would glow sometimes, and sometimes they didn't. If the glow light didn't go on when I first tried to glow the plugs, I just kept trying until the light went on. You could also try a junkyard where you may get lucky and find one for cheap.

If you ever get desperate or get stuck somewhere becasue the relay failed, it's always a good idea to keep a piece of 8ga wire in the car. You'll be able to glow the plugs by placing one end on Plug #4 and the other on the positve battery terminal. It's a pain, but could get you through until a new relay can be purchased.