You will almost have to get out a spreadsheet to line item compare what is included in each venue and what would have to be rented separately. I’d assume a caterer will include plates/tableware etc in their quote. Does their quote include tax/gratuity because those can come as a nasty surprise. But then if you self cater you’ll undoubtedly have to rent your plates glasses etc.
But to kind of start to compare:
Venue 1: 2200+6000 (no booze) all day so relatively more relaxed set up / tear down. 8200/75 guests is ~110 per person (plus booze plus tax / gratuity)
Venue 2: 4600 (no booze, no food, no staff, no plates/tableware, plus probably some tax and maybe some kind of service charge)
Difference: 8200-4600= 3600 / 75 guests = $48.00 per person. Sounds like a lot, sounds like a great way to save money.
But wait ! Consider food costs. When will you buy it? Who has enough refrigerator room to store things? Renting chafing dishes, methods to keep hot food hot/cold food cold. Food safety so that you don’t end up with sick guests. What’s your timeline to make the food – who will do it? Will you need to hire day of helpers (yes, you will! – no family member wants to be managing a kitchen rather than party with you on your wedding day.) How will you transport food to the venue… what vehicles, how many trips, what time during that 7 hour rental when you’re getting ready and others are setting up. Who will carry in all of the heavy crates of food, who will dish them up, display them, keep them full? Who will carry in stacks of plates and glasses? Who will scrape them and rinse them at the end of the night? Who will get rented things back to the rental company? Who will get food that needs to be heated into the oven at the right time? How many ovens do they have – enough space with enough heat/output to heat though multiple trays of cold food (it’ll take longer than say cooking one pan of lasagne at home because you have lots of cold mass that needs heating.)
Maybe think about your mom on Thanksgiving – cooking a dinner for maybe 10? At the end of the day she’s probably ready to crash — started early, cooked all day, managed timing of food to pull it together at the right time, then had to clean up after it’s all done. And she didn’t have to haul stuff to and from a venue load/unload it several times. Self catering can be done (though it would be easier for someone who routinely throws back yard parties for large groups) you do save money, but spend a lot of time and physical effort and worry to accomplish it. Paying for the food is just a small part of what you’re paying for when you hire a caterer.
The questions I’ve asked are but a few of the ones you’ll need to figure out if you decide to do this.
Because I like to cook, I’ve ‘catered’ several holiday lunches and birthday parties for 30-40 people. It’s a labor of love I was able to do great food for minimal cost … but it was very physically demanding. Cooking, storing, loading, traveling, unloading, setting up, refreshing, tearing down, cleaning up, loading, returning home, unloading at the end of a very long day. So on one hand, I can’t say “don’t do it” because I have. But also understand I won’t be doing it again any time soon, and doubling the guests to 70 would have me saying no way.