As much as they like. There are no limits, either for individual lobbyists or their clients. In 2005 the US Chamber of Commerce led the field, spending nearly $39 million on lobbying for that year alone. In the first half of 2006, the Altria Group (formerly Philip Morris) spent the most, $6.8 million.
Keep in mind that lobbying costs are less exact than campaign contributions. When clients hire outside lobbyists, the full value of their contract must be reported. But the cost of an organization's in-house lobbying is only an estimate.