Ours cost £6k for 100 people (think size is important - will obviously bump up the cost!), but our hired food was more expensive (£1.5k) and we also had a free bar and our venue was £1k.
We didn't have a dj, just spotify and borrowed lights and speakers - everyone still danced loads!
I made a lot of decorations out of cheap / free stuff eg facebook local for free jam jars, put fake tealights in them. Blew up normal balloons, borrowed fairy lights. Also bought cheap ivy from amazon that looked very effective. Sold a lot of stuff afterwards.
A lot of flower places offer buckets of flowers that you can diy - that's what we did and then then totally unexpectedly my aunts and female cousins turned up to help the afternoon before, so I have lovely memories of watching them all flower arranging and chatting to each other. Gave the flowers away at the end.
We bulked out the expensive food with stuff we bought ourselves eg nibbles, salad, cheese, bread, evening food. I also bought our fizz from Aldi.
Our cake was a wedding present (and pudding).
We didn't have a photographer and offered prizes (posh chocolates) for the best photos - we now have hundreds and loads are really nice.
Borrowed outdoor toys for the kids, bought bubbles, paper pens, sweets etc. Kids had a whale of a time.
Hired tablecloths (most cost effective thing) but bought cheap burlap as table runners and sold those afterwards.
Didnt do favours, seating plan, programmes, sent invites via email apart from very elderly relatives who got a card invite (bought 1 pack from paperchase).
Congratulations and have fun!