You can have a small wedding ceremony and you can wear whatever you want. Regardless of whether you choose to have a religious/church ceremony or a civil ceremony.
Marrying in a church doesn't require you to invite a lot of people and spend a lot of money. Both sets of my grandparents married in their respective churches in their best Sunday clothes with only 2-5 guest present, then they just went out to lunch afterward at local restaurants.
If you forego the paperwork, though, then it's not a wedding. There's no point in having a fake wedding where you don't actually get married. And a church/temple generally will not perform a fake wedding ceremony for you. You will need to produce a marriage license in order to actually get married. Otherwise they're going to ask WTF you're even talking about and tell you to come back in a few years when you're actually being serious and not just a couple of teenagers screwing around.
Yes, a church generally charges a set fee or asks for a donation in exchange for performing your wedding. They have to pay the electric bills somehow, you know. They're letting you use your facility and they're giving up their time to perform a service for you, so they shouldn't be expected to do it for free. If cost is truly a hardship (not just you being cheap) then talk to them about a reduced fee, or a payment plan, or volunteering your time in lieu of giving them cash.
And honestly I found that the "suggested donation" from my church was WAY less than what a banquet hall/hotel/museum/etc. would've charged me for the venue, chairs, lighting, sound system and music, the officiant/minister, etc. So in many cases a church is actually less expensive than a rental venue.