It can add up to lot more than you think.
You should search this website for information.
There is at least one detailed thread on AAM dealing with this.
Basically, you have to put something in place of the wall to continue the support.
The actual cost will depend on many factors:
the type of construction
whether the wall is internal or external
whether the work will disturb services / pipes
whether or not the working space is constrained
whether another loadbearing wall abuts it or vice versa
how high/deep the arch/beam downstand is to be
how wide the span between supports is
new construction details [timber, steel or concrete?]
After sorting all that out the only cost worth knowing is what a builder will actually do the work for.
and...
If you don't take the advice of someone like a structural engineer to determine beam size and bearing required at the ends,
then
Add a contingency for later repairs of diagonal movement cracks running upwards from the top corners of the new opening.
Not all builders make a mess of things when they are not following drawings and specifications, but enough do.