A small extension to a dining room could cost as much as a large extension!
Size isn't always a cost factor as site access, digging, pouring conc etc on a small job can be labour intensive and therefore expensive.
I bought a steel beam last year, 5m (15' 4") long. It was 8 x 4" steel I beam, cut to size & delivered cost €300. Let your fingers do the walking - get some quotes.
The €1300 includes labour, 2 men for 2 days work & Engineers fee.
A steel girder or I beam is required once the span goes over a certain length. Its size is specified by a Structural Engineer with full PI Insurance.
"An acquaintance told me that a steel girder could cost up to €8000".
It might cost €8k if you are building a bridge! Take your acquaintance with a Pinch of salt.