As of right now, June 2nd.
https://www.southwest.com/html/travel-tools/
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Prices for our October trip went up just over $150 if we fly out of GRR from last time we flew with Southwest (and now have to connect in MDW). Might just drive to MDW and fly from there. Still cheaper than Delta or United for the same time frame either way.
Have you checked the cost of flying out of Flint? Sometimes that is a lot cheaper than GRR and MDW.