Good rifle
100-425 USD
I have had good success with Federal rifle ammo.
25-180 or so
.284
No, they are not interchangeable.
300 or so
Lacking any more details; 10-1000 USD depending on specfics
ar-15
Last year I paid $550 for a very good condition used Rem 650 .270 win.
Long
Many do, but, not all the time. You will have to call to find out.