It shouldn't be that much, but who knows? Call around. My brother just got one for his Metro and it was $125. I think they are about $70 for the Mustang.
You may want to try to clean the one you have. I think you said you did that in the other thread so I'm not sure if it's cleanable at this point. If you only did a basic cleaning, though, you may want to really focus on cleaning it to save the money. Concentrate on the valve opening. Find the best product you can for disolving carbon. You may even want it to soak, just doon't soak the diaphragm.
Just to see for sure, you may want to bolt a plate over the holes where the EGR goes. You'll probably have to fabricate it, but it shouldn't be that big of a deal. If performance improves, you know that you can just get a new EGR and fix it. If it doesn't, you can eliminate the EGR as a problem.
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351W 89 Mustang GT Convertible