Actually, you don't need a whole castle, just a part of one for a few hours on the weekend. Better yet, a weekday, when rental fees are usually lower.
Try starting with your park district or state parks department. Tax-supported venues generally rent for less than private businesses. You don't need to settle for picnic shelters or campgrounds for a secluded atmosphere, either. How about an arts center, nature center or restored farm? You can rent part of a mansion or historic landmark -- the courtyard instead of the carriage house, say.
On the downside, these places may accommodate as few as 50 people. You might also have to choose a caterer from a "preferred professionals" list and purchase liability insurance.
Another option: If any family members or friends belong to a college or university club, they might be able to snag you a stylish setting at a member's rate.
Next: how to make an elegant entrance.