BP said the cost of the clean-up in the Gulf of Mexico now amounts to $3.5bn, up from $3.12bn a week ago and $2.65bn two weeks ago.Some brokers estimate the Gulf disaster could cost $70bn. However, bid and asset sale rumours are continuing to lift the firm's shares. US oil giant Exxon has started to clear the political hurdles ahead of a possible bid for BP, reports yesterday suggested. There is no indication yet that Exxon intends to make a move, but the firm has sounded out the Obama administration and been told the US government would not stand in the way if it did go ahead. Another firm, said to be Chevron, has also been told the authorities would give their approval. A successful bid by Exxon for BP would create an oil company worth more than $400bn (£265bn). BP is also said to be is in exclusive talks to sell up to $12bn of assets to US firm Apache, including a stake in the huge Prudhoe Bay field in Alaska. Apache approached BP with the plan a few weeks ago. Discussions are under way over the structure of the agreement and if other assets could be included. BP wants to raise $10bn from asset sales to help meet the cost of potential compensation and litigation from the oil leak. In addition, BP is said to be in talks to sell its $9bn stake in Pan American Energy, an Argentinian oil producer.Investors also cheered news today that the relief well that will seal the well completely should be finished within the next few weeks."Although uncertainty still exists, the first half of August remains the current estimate of the most likely date by which the first relief well will be completed and kill operations performed," the group said in this morning's statement.