We got married at Faith Des Peres Presbyterian in Des Peres/Frontenac. Not sure how much it cost, but you don't have to be a member, the clergy and wedding coordinator are great, and the area is superb. Give em a holler, (314) 432-8029.
I'm not sure what type of church you are looking for, but my husband and I had the same problem. That said, I called about 20 different churches (all Catholic) and we found the beautiful St. James the Greater in Dogtown which was very accomodating and I believe it cost $500 if I remember correctly. It was, by far, the least expensive church we came across on top of the fact that it didn't require you to be a member. I can't really speak for other denominations. Good luck!
Are you looking for a particular religious donomination?
Honestly, there are quite a few Churches that offer Weddings at their location, as a 2nd income. Therefore, they are will to Marry most anyone and you don't have to be a member. Unfortuately, the Fee is hard to get around. $700 is actually lower then some churches. I do wedding planning and my last bride paid $1200 just last month.
What area in St Louis are you leaning towards?
Are you set on holding the ceremony in a church? I got married at the Hearth Room at Hawken House in Webster Groves last month. We held the ceremony outdoors in the garden and the reception in their renovated barn, it was beautiful. You can also hold ceremonies indoors where we held our reception. They cost $850 for the whole day (ceremony and reception); I think its $950 on Friday/Saturday (we got married on a Sunday). If you are set on a church wedding, this has likely not been helpful. But if you are flexible, check at Hawken House and look for other sites that do ceremonies and receptions (wineries often offer similar setups).
My profession is to help couples find event locations including places of worhship that allow non-members. The average cost for a religious ceremony in the St. Louis area is $850. There are places that are less and places that are more - but the geographic location, the size, and condition of the facility is not perfect.
Basically, you get what you pay for.
Click on each tool to see a sample.