Train from London to Paris

Buy in advance. If you're travelling in mid September the tickets will be expensive by now. I'm not familiar with the exchange rate, but 500-600 CAD sounds very expensive. Unless it's the price for two returns. Make sure you buy them direct from the company that actually runs the trains, www.eurostar.com , and avoid third party resellers.

Those prices could easily be the best available at such short notice. There would have been much cheaper tickets available several months ago but they are most likely all gone now.

Just checked the US Eurostar site, and for your dates converting to CAD, $500-$600 looks pretty good. You should buy ASAP as prices will only go up from here. That is return tickets for 2 adults.

PS, your return on a Saturday is the most expensive day of the week, return on the 15th or 17th saves a little bit.

The really expensive part of the journey is the Monday morning departure. Go in the evening and it is £100 cheaper each, roughly.

Thanks so much, didn't realize the prices change so much closer to the date, but that makes sense. On some 3rd party sites I was seeing a reduced rate for seniors, but on the the eurostar site I'm just seeing 1 price for adults... strange?

I think the Senior rate would exist if you were on a Senior Eurail pass- that is travelling further in Europe on a rail pass.
There is a quota of pass fares on each Eurostar journey.
But on a point to point Eurostar journey there are no senior discounts that I know of.
I hate to suggest it, but could fly out and Eurostar back be more cost efficient?

Air fares tend to follow Eurostar as that is where the capacity is. Mid morning flights with BA & AF seem to be around the same price. The cheapest nearest option is possibly Vueling @ 12:50 from Gatwick to Orly. €41 base fare but if add-ons are required it would possibly push towards £100.

Last minute, these tickets would be almost CAD $1400, so that is why you should buy now.