NS - if you're not happy with your purchase and haven't worn them outside, take them back and either swop them for something else or ask for a full refund - just explain you've made a mistake - you thought the shoe was neutral but it's a support shoe.....
I think this shoe was 2006 - 2007 and I have seen them much cheaper on line - £39.99 as opposed to their RRP of £74.99. I don't think the shoes condition deteriorates until you've used them for 100 miles or so! And I've never used this shoe, though Mizuno do have a good name and I've a couple of running friends who always use Mizuno's.
As to being "ripped off", unfortunately it's the shops decision as to what price they have their products at and ultimately your decision whether to buy or not - why did you decide to change the model of your shoe? Was it discontinued? A decent shop should swop or refund though and I think that if you are within a certain timespan they have to refund if you are not satisfied, though if you no longer have the receipt you may have to settle for a swop. I'm not well up on retail law.
Good luck