Run 5-10 of these. Time the plotter run time, and the weeding time. Determine what you need to make on an hourly rate and convert that through your pricing of 100.
I find trying to quote small stuff like this by the square foot usually hurts me in the end. To make it realistic, I run trials and then determine the quote.
It's a bit of a handshake agreement with the client at that point, as I usually would have their business already, based on a "soft quote."
As for offset, both your machine and software should have settings. Run tests on squares, stars, letters, and run a few different options.
Your blade could be dull or cutting strip chewed up, but I'd say it has more to do with offset.
I run small stuff like this all the time on my Graphtec at 0 offset with a 45 degree blade. And very light overcut added through Flexi.