We aren't a tour operator. Instead, we represent local European tour operators. The local European tour companies we partner with have much lower prices than their North American counterparts, and we don't mark anything up.
Why are their costs lower? Low overhead, no need to transport staff and equipment across the ocean, no North American-style marketing budgets and salaries, and a larger client base, which allows them to amortize costs across more clients.
The lower prices don't mean lower quality, though; you're often staying in the same hotels as travelers who paid two or three times what you did.