I think I waited too long to purchase tickets. Was planning on a day trip to Paris October 4th. Tickets seem to be $275 RT. Any tips for discount sites or any knowledge if the prices ever go back down?
Assuming you mean Eurostar. Buying earlier might have saved you some money but as it's a Friday/weekend you are just going to have higher prices unless you take a very early or very late train. My experience is the fares seldom go down the closer you get to departure date - in an unexpected travel delay situation I paid much more than your price that for tickets the just the day before travel.
More info from Eurostar on how to get "cheap" tickets: https://www.eurostar.com/us-en/deals/cheap-eurostar-tickets
I agree with CL. Eurostar is not cheap to begin with, and Fridays are never
good days for cheap fares.
However, if you are willing to be flexible and take a day trip to Brussels
instead of Paris, you can still do it on Eurostar for ~$150 or a bit less RT.
If this is a day trip you will want the longest time possible there, so take the 0601 out, and then most trains back, and it is £168 return (US$192).
No the fares will not go lower. No there are no discount Sites.