I have just priced up tickets for the 5 of us (2 ads, 3kids) on the Eurostar for Feb half term and is about £600ish is that about what I would expect to pay? I keep reading about people booking packages inc hotel & train for a few hundred pounds including accomm and travel so wonder if I am doing it wrong!!
Have you been looking at the prices on the Eurostar website?
If you play about with times and routes (ie not necessarily the direct DLP service) you may be able to find it a little cheaper.
We are going out on the direct service, back via Lille, and it cost £45 each, each way, so a total of £180 for 2 of us. When we booked, that was the cheapest option. But if we'd chosen different train times on the same days, the price would have gone up.
Also, I think Eurostar works like the airlines, with prices changing according to demand. I've just checked and if I was buying our tickets now it would cost £67.50 each, each way, so a total of £270. For your dates in Feb it may well be that they know it is half term and, just like the airlines, have increased the prices accordingly.
No doubt people do sometimes get special package deals, but they have to be able to comply with the terms and conditions of those deals such as specific travel dates. Travel companies only offer deals to get bums on seats and fill rooms which would otherwise be left empty. Half term is unlikely to be a problem for them.