The die# that matches Bonnie's MQ is 55027 (finishes at 3"0
http://www.accuquilt.com/shop/go-tri...ed-square.html
There is also a smaller one, #55410, this one finishes at 2".
And there is a larger one, #55409, that finishes at 4".
On my Mariner's Compass, I've put myself on the "Fabric Moratorium", so my colors won't be the same as what's in the pattern... I really DO need to dig in my stash and see what I have already that will work instead of buying fabric.
Since you said you needed someone to "make me think twice before ditching all that money", here's me telling you to think four or five times. Don't waste your money on that method!! The Accuquilt dies really ARE the better choice.
I know some people think there's a lot of waste with the dies, but... when I cut inaccurately, and it's SO BAD that there's no way to salvage it, I've wasted MORE than if I had used the die. If I'm doing an entire quilt (like the Mystery Quilt), then I use the dies constantly. If it's something smaller (like the Moda Frivols that came out last year), then I just use my regular rulers and rotary cutter.
Just my 2 cents worth, anyway...