I think Lucius may have been playing very high speed chess when he picked the amount. The point isnt to have Harry in debt to him, the point is to afford ex-deatheaters loyal to Lucius the oppertunity to trade in a blood debt to Harry for a monetary one to him.

I don't think so. It's clear from his reaction that he did not want Harry to accept the trade:

It was clear that Lucius Malfoy had not been expecting that reply.

And later:

"I withdraw my offer!" shouted the Lord of Malfoy. "I will not accept the debt to House Potter in payment, not even for a hundred thousand Galleons! The girl's blood debt to House Malfoy stands!"

Indeed, it is a taboo tradeoff from Lucius perspective. He has traded justice for his son for money and the resolution of a political (though not moral) debt to Harry. He picked the amount because he thought there was no possibility that Harry would accept such a large amount.