The cost depends on what you get, but they're completely optional. Your guests won't miss them, and a lot of people would leave them behind. The exception is edible favors, people always appreciate some kind of late night snack as they travel home.
Of all the weddings I've been to in the last few years, only 2 had formal "favors" (and only about half had any type take home thing i.e. name tags, centerpieces up for grabs, etc). The weddings with favors had a LOT left over - no one took them.
When my mom asked me if I was going to do favors at my wedding, I told her "yeah. Free food, alcohol, and party music" and it was left at that. I personally hate party favors at weddings. It's fun at a kid's birthday when everyone gets their own squirt gun. It's less fun at a wedding when the favor is a blaze orange can coozie with the couple's wedding date on it.
Save your money and put it towards the guest experience that they remember long after the wedding: was the food delicious/hot/plentiful? did they have to open their wallets for a drink? was the dj able to keep guests entertained on the dance floor?
We have attended weddings that have all types of favors and none at all, and the preference is to not have any. Even with the edible favors, the couple’s parents couldn’t give them away at the end of the night.
We are doing his and her favorite candy for each guest. We have one thing in our favor though for left over favors, we are getting married about a month before Halloween so any left overs willbe passed out to trick or treaters ( no dates or names will be on the candy).
Rather than having traditional favours you can have items of decor or food that the guests can either enjoy at the wedding or take home.
One wedding I went to had mini wedding cakes for us to all take home and I really enjoyed the cake the next day! But it must have been pricey. Another wedding item I kept for ages was a menu card that the bridge's grandmother had decorated with an illustration for each individual guest based on their hobbies/personality. The personalisation was lovely.
We had our tables decorated with small house plants and they all got taken, and I know a number of guests still have them thriving in their homes! We also decorated the tables with miniature easels that had photos of locations meaningful to us. It didn't cross my mind that the guests would take them but they did which was great.
Something else I've seen done a lot is gifting the leftover wine bottles and floral decorations to guests who've helped in some way for the wedding. It's also possible to have little boxes for people to take away left-over cake slices, which can be nice. (I know I'd take one!)
You definitely don't need any specific "favors", and with a limited budget can focus on the day itself.
We had our snacks and drink served in hampers the guest could either take home or consume on the spot - boxes of specialty biscuits, mini bottles of bubbly and spirits, various snacks etc + a pen knife/bottle opener to help with opening the various snacks and bottles. This is a bit unusual but it was to cope with the covid regulations which required individually packed food items and minimising contact between serving staff and guests, plus items that could be transported to any location at the last minute based on the changing rules and something for the families that didn't feel comfortable staying for the social despite the social distancing. It worked really well though and the guests could also keep the baskets and reusable cutlery which served as a picnic set. I know at least one family who used them for ages for their children.
We are not doing favors and I think it's becoming more and more common not to have them. I can't tell you how many random coozies, shot glasses, sunglasses, etc. that have "so and so's bachelorette" or "he and she wedding date" on them that have never been touched after they made their way back to my house.
It's a sweet gesture but 9 times out of 10 any favors you give will be left at the venue or tucked away at their house never to be seen again. A lot of people do edible favors which are great and super cute but can add up really quickly!
If you're wanting to do something DIY you could make some little truffles, cookies, or candies of some sort!
If you don't have the extra money then don't bother with favors. They get left behind, ignored, or thrown away more often than not, which then leaves you with a bunch leftover and probably feeling some kind of way about putting all that money towards something nobody acknowledged.
If you do have a budget for something extra, you could opt for a higher-tier bar package, late night snacks, a photo booth, a caricature artist, or something else like that. Guests will remember their experience WAY more than party favors.