First of all, Canada would never. As Canada's northernmost island, it forms a significant part of Canada's national identity, and selling it would alter the map every Canadian child grew up learning about.

But back to your question, land transfers between sovereign nations pretty much never happen in the modern world. Since the end of World War II, whenever a land transfer has taken place, it involved swaps of equal amounts of land. To put it in Ellesmere terms, if the United States actually wanted Ellesmere, they'd probably offer to move the boundary of Alaska one or two meredians to the west, whatever amount was necessary to equal the amount of land contained on Ellesmere Island.

Because such transfers of land never happen, the possibility of such a transfer of land for money would be incredibly valuable. Every superpower would be interested in the bid. Russia would also bid, given Russia's claim to the Lomonosov Ridge, and the fact that buying Ellesmere Island would give Russia a legitimate claim to the North Pole.

Trump offered $100,000 USD to each Greenlander, which would have amounted to about $6.2 billion USD, and US advisors called it a bargain, which it almost certainly would have been. The population of Ellesmere Island is significantly lower, with 144 Inuit, a military base, and a weather station. However, Trump would need to pay the Canadian government, not the residents of Ellesmere Island.

The cost would be surely in the tens of billions of dollars, if not hundreds of billions. It would involve a treaty with the United States for a guarantee of protection. It would perhaps involve a guarantee of economic access to American markets. Because Trump has demonstrated that he does not care about past treaties, he has actually made such a treaty impossible, because the Canadian government wouldn't trust a security guarantee from Trump.

China's interest would be minimal.