You could try making your self a template out of template plastic or cardboard and use that to mark. Problem with the cardboard is you can't see through it to know if you are the desired distance from your first mark.
Are you frame quilting or sit down quilting and what kind of rulers are you looking at that cost that much? A single ruler from Quilted Pineapple only runs around $30
http://thequiltedpineapple.bigcartel.com/
Rhonda's medium size crescent is around $20
http://quiltersrule.com/ProductThumb...22&tier=retail
These are both for frame quilting but you may be able to use them for sitdown.