Scottish Conservatives, who obtained the figure using Freedom of Information, said it was a “kick in the teeth” for islanders forced to rely on the crisis-hit ferry network.
It would have cost around £75 for Mrs Slater and her entourage to go for CalMac.
The co-leader of the Scottish Greens, whose party urges greater use of public information transportcame under fire last month after it emerged that she and her officials chartered Mallaig’s catamaran.
The Scottish government said at the time that using Western Isles Cruises’ 42-passenger MV Larven would “maximize” the Circular Economy the minister’s time in Ron.
Ms Slater and seven other people were seen boarding the ship ahead of their five-hour visit on May 12, when the £9.40 return fare on CalMac for all of them would have been £75.20.
CalMac’s schedule also showed that he could have spent five hours and ten minutes in Rum using its regular service that day.
Slater declined to answer questions from the press about using a private charter while boarding.
Tory transport spokesman Graham Simpson said: “This revelation is yet another kick in the teeth to Scotland’s betrayed island communities, as well as the taxpayers footing the huge bill for the SNP-Greens ferry scandal. Lorna Slater clearly has no shame.
“The government of which you are a member have created chaos on Scotland’s ferry network due to their incompetence and yet thought it appropriate to avoid a CalMac ferry at less than £10 a head return, to shell out £1200 of money public on a chartered boat. to take her and her team to Rum.
“He is completely deaf to the plight of the islanders, who have to make do with a pitiful ferry service. It is also the height of hypocrisy for a green minister who always demands more use of public transport.”
At the time of Ms Slater’s voyage, the Scottish government defended her by saying that she “was traveling with members of the Isle of Rum Community Trust, NatureScot and government staff, on a charter operated by Western Isle Cruises”.
A spokesperson said the letter would “maximize time on the island and support a small local business.”
However, the Scottish government was more concise today, telling the Scottish Sun: “The trip was organized by NatureScot, who managed travel arrangements to the island.”
NatureScot, the agency for which Ms Slater is responsible, confirmed the cost of £1,200.
Mrs Slater visited the Isle of Rum to discuss the future of Kinloch Castle after the town’s financier and former Conservative donor Jeremy Hosking withdrew a bid for the Edwardian building, blaming Mrs Slater’s intervention.
Speaking after his visit, Steve Robertson of the Isle of Rum Community Trust told the BBC the dispute over the boat was a “storm in a teacup”.
He said: “Lorna Slater is coming in to do a very important meeting to try to move things forward for the community.
“It makes people feel disappointed that that is the story when for us a taxi charter boat is a normal part of island life. We have to use them to make the island sustainable.
“She can take the ferry service if she wants to have the meeting on the ferry and fit in with the very limited options for spending time in Rum.”