I would like to get a spotting scope for mirage reading only when shooting F/tr. Do I really need a high dollar spotter for doing so?
That is the sole reason I use spotting scopes and NO you don't need a high priced one.
You want to a scope for reading mirage, it does no good for me to tell you band X will work, Brand X may work for me but chances are my eyes are the same is yours.
There is just one effective way to pick a scope for mirage. Pick and day where there is no mirage, or you can see any mirage, its there whether you can see it or not. The go to a large sporting good score. (Cabalas, Scheels, Bass Pro, etc) Tell the clerk what you are looking for, and that you want to check them out. So they will take you and a few scopes outside, and look though the scopes, several, until you find one that you can read mirage. Don't use the paved parking lot, use a gassy field.
You'll see the difference in scopes, and not just in high priced scopes. The last one I bough was a Scheels, and it was Leopold which only cost about $275, it works, BUT DON'T buy a Leopold because I said mine worked. Chances a different brand will work for you.
What ever you do, don't buy a spotting scope to read mirage with out looking at mirage through the scope. Don't shop for spotting scopes on a hot humid day. A cool dry day will make mirage harder to detect, that's when you want to shop for scopes. Anything can read mirage when its hot and humid, But we don't always shoot in those conditions.