Day trip to Paris?

Depends on when you are going and buying tickets. From the US Eurostar website:
If it is for 1 week from now, it will cost $904 US
If it is for 4 months from now, it will cost $264 US.
Those are the prices if you buy today, and will give you about 12 hours in Paris.

Is there something specifically you want to do or see in Paris, or do you just want to do it because you can?

I've done the day trip both ways -- London to Paris for one day during a trip to London and Paris to London one day during a trip to Paris. Was it worth it? For me, the answer is yes. It's no big deal. You buy your ticket, show up at the train station, go through passport control and relax on the train until you arrive at your destination. I do not do the day trips to try to visit/tour the city but rather to do something specific there: for example to take someone to see the Eiffel Tower who had that as a bucket list item but had never been to Paris before and did not think she would ever get to Europe again. The day trips are great if you have already visited the city and have a specific thing that you want to do or if it is your first visit if you really limit what you expect to do. Ideally, I'd keep it to one must-do thing and anything else that one gets to do is lagniappe. Don't forget the time difference. I find that one hour difference a little bit tougher on the schedule for the London to Paris day trip -- you leave London at around 7 AM and do not get to Paris until after 10 AM.

I haven’t done it. I’d rather do a day trip closer to London.

Please consider that Eurostar recommends arriving at the station in London 90 minutes prior to departure and 90 to 120 minutes in Paris. So that’s 3 hours of processing and waiting time plus 4.5 hours train time. 7.5 hours total. For me that’s a waste of valuable vacation time. I’d rather plan an overnight or a few days in Paris.

I have done it. But I was living in London at the time. I had already been to Paris a few times. And I knew exactly what I wanted to do. To maximize time I used taxis to get around. I went from train station to department store. I wanted to buy something specific I had been missing from my last trip to Paris. I went to the Museum of Decorative Arts next to the Louvre as it had just reopened after refurbishment. I had lunch at nice restaurant nearby. I had made a res. I went to a wine tasting. I had made a res for that too. I had a walk along the Seine. I had taxi back to train station. It was a long day but in that context it was worth it for me. If your whole trip to Europe is one week and you are based in London I think there are soooo many enjoyable things in London or nearby that I would not take a day just to get to Paris.

I've considered this a few times, do a realistic equation of ALL the travel times involved, and the reason you're going to Paris. If you've never been, it could be a cruel tease of all the things you can't see. If you have been a few times, just pick one site or two and slowly enjoy.

Man in Seat 61 has done this trip, as have many other train travel vloggers.