I always buy the specialty ruler. Saves time, frustration and wasted fabric. If I enjoiy the pattern, I will make it again. If I'm not in love with it, I sell the pattern and ruler on Ebay. If you don't want specialty rulers, don't buy the pattern. There are lots of patterns that use the rulers you already have on hand.
What is frustrating is buying a pattern that doesn't advertise that a specialty ruler is required. I really resist buying a pattern without a list of requirements without asking the quilt shop staff first.