I'm looking at fares for Paris-London for Fri., Sept. 18 and have checked both on raileurope.com and eurostar.com. The one-way fare for one senior on each is $96 each and the return fare (I've always understood it can be cheaper to buy a same-day return and not use the 2nd half) is $104 on raileurope, $103 on eurostar. Obviously, the return-fare theory is not working! Should I try putting in another home country, such as UK or France? I put in US.
Thanks for any help.

How are you "looking" at eurostar's website? Eurostar itself typically will not start bookings until three months in advance of the travel date. This is when you should be looking at pricing the trip. Rail Europe will likely be charging a premium if you go though that route.

MrNuke,
I was typing in the date on Eurostar. So you think it would be best to wait until 3 months out and then check? I've read where the best rates are found far in advance, and that they increase as the date nears.
And is it generally true that return tickets are less than one-way?

Tickets bought in advance will be cheaper (usually) than those bought at the last minute. But they don;t start selling until 90 days out. Also, be aware that the same as airlines different days of the week and different time trains will be different prices (high for those that sell out, lower for those with fewest riders).

To get tickets directly you have to go to www.Eurostar.com - and they are not selling tickets past July at the moment (the calendar won't go past there).

Yes, wait until 90 days out and yes, it's generally true (IME) that RT tickets are less than one-way. My son and I bought RT tickets last August and just discarded the return portion because it was so much cheaper to get RT tickes. They still weren't cheap, but the price differential was considerable. Maybe others have better suggestions - I'm no expert on this, having only done it one time (but asked Fodorites for suggestions before doing so).

Thanks to all for your helpful responses. Will try again in a couple of months!