Repeat after me: Do not worry about insulting the seller.
And the problem with the scenario you describe is that it really doesn't matter much what the building costs were 2 years ago. That's ancient history.
Honestly, I don't know what the markup is. But let's just make up a scenario. Let's say it cost a builder $300,000 to buy the land and build the house. And it's on the market for $450,000. I understand your question: What might you offer that the builder would accept? So you might say, maybe, $350,000 or $375,000. And maybe that's a fair answer.
On the other hand, two years ago: The builder has had two years of carrying costs. Throw in two years of interest and taxes on $300,000. That alone might be $70,000. So, using your scenario, $350,000 wouldn't be a good offer anymore; it'd be under the builder's costs.
On the other hand, let's say property values have declined sharply. When the property first came on the market, it might have truly been worth $440,000. So it was just slightly overpriced, which is why it didn't sell. Now housing prices have come down 40%. It's only worth $250,000 approximately. There's no way that house will sell for $350,000. And you'd be making a horrible mistake offering $350,000.
Having an idea of what the builder paid will not help you. And it'd be the builder, not you, who'd be insulted or not insulted. Unless your name is Emily Post, that shouldn't be a consideration.
The only thing you need to know--repeat after me, the ONLY thing you need to know--is how much the houses are currently worth in today's market. Then you make an offer no higher than--and in today's market almost always lower than--today's value. If it's accepted, fine. If it's not, that's fine, too.
Hope that helps.