Hey! I looked into Hartwood..... it has been awhile since we talked to them but from what I remember was that the prices seemed pretty reasonable... they charge you to rent a tent from them and then a fee for renting the space but I THINK past that you can use any vendors you want, bring your own alcohol etc. The reason we did not end up there is we were told that weddings needed to end and be cleaned up by 7:00 when the park closes. (I thought that seemed strange so maybe things have changed or maybe we were given wrong info).
If you are looking for outdoors I would look into the Mayernik center (at Avonworth Community Park... but this might be more 'country charm'), the Pittsburgh Zoo has an outdoor garden tent (this is the venue we chose), I was at a gorgeous outdoor wedding at Frick museum , and I believe the Pittsburgh Center for the Arts has outdoor space.
I looked at it as an option, it's really beautiful. The downside was having a back-up plan if the weather was bad. Their prices are listed on a website somewhere, a quick google search should provide what you're looking for.
Re: Hartwood Acres Mansion
GL!