Sorry I have no specific advice re cost, but have some experience of a lift in a private house. If you can afford it at all, get one. I had two elderly relatives, who, because they had a lift, were able to continue living in their small, but beloved home, until the very end. One of them was totally immobile, the other with a weak heart.
What it meant was, that caring for them was easy, no hoisting onto stair lifts, they never feared falling down stairs, and they did not have to sacrifice any living space on the ground floor.
I know they paid a substantial sum for it, and they were not wealthy by any means. They had it for 9 years, and when asked had they thought it expensive always replied that it was worth twice the price.
I have just built a house and have left a space for a lift should my parents ( or indeed myself) need one in the future.
I should add that this was in the UK, where they are not as rare as in Ireland. Maybe have a surf around some sites in the UK - probably not much help for Ireland, but you never know what useful information you might pick up. Don't forget to look in Northern Ireland.
Best Of Luck.