I believe the pavilions at all Murfreesboro parks are $50 to reserve for either the morning or the afternoon. You can make the reservation over the phone, and you pay in person at the parks and rec office at Barfield Crescent Park.
https://www.murfreesborotn.gov/190/Old-Fort-Park If you will go to their website and click on that park, the prices are listed for both the large and small pavilions for full and half days!
I would have to say that this would be the best place to have a birthday party. As for the price, I'm not sure.
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