The figure has risen from the £170,000 previously estimated to be necessary by LI chief executive Alastair McCapra.
Transferring the library and archive to another organisation, along with five redundancies, were proposals planned to make the savings. But more drastic action is now needed, McCapra said this week.
"The library and redundancies are only part of a much wider picture," said McCapra.
"In October (when the consultation began) the consequences of the economic downturn weren't fully evident.
"Obviously nothing has happened since then that would give us grounds to be more optimistic."
In addition to lost advertising revenue, which makes up around a third of the LI's income, the organisation is now facing a doubling of its rent at its London headquarters.
The LI is now considering moving to cheaper premises, possibly outside the capital, said McCapra.
A meeting of past presidents and fellows is scheduled to take place today (16 January), with an EGM for all members planned for 22 January. The issue to raise the most concern so far is the possible transfer of the library and archive to another organisation, and Garden History Society chairman Dominic Cole is leading a campaign to ensure it remains with the LI.
Archivist Annabel Downs has already received a letter of redundancy from the LI, a move which has angered campaigners.
"We called for no decision on the post of archivist or moving the library until the EGM had been held," said Cole.
"It was a very clear request from a number of senior members of the profession so I am left with concern, uncertainty and lack of trust."
Four "fairly firm" expressions of interest have already been received by organisations willing to take on the library and archive, said Mc-Capra, and a committee led by LI vice-president Jo Watkins will be set up to look at the offers, and any other options.
Past president Brian Clouston has also been campaigning to retain the library and archive. He said: "The institute should be listening to its membership.
"People care passionately about the library and archive - it is our history and also belongs to future members."
McCapra said: "The council is obliged to consider carefully the expression of views from the EGM but it is not necessarily bound to follow decisions made at the EGM.
"For understandable reasons people have focused on particular aspects like the library and archive or redundancies, but it is not safe to assume that anything we do will carry on the way we do it now."