Your best bet probably should be a Minolta AF 100-300mm or 75-300mm.
Both are AF.
Both are about US$200.
Only if you can find a good copy.
If you need to experience superior power, then a mirror Minolta 500 AF (Yes you got it right, AutoFocus, unlike most in this class that are manual). This should be about $500.
If you don't care much about IQ of your pictures, you can always try cheap manuals such as this type
L I N K but then picture is going to be very soft.
Better than this in same 500mm class would be:
500mm non mirror You need the right T mount though.
500mm mirror
Again don't expect too much about image quality.
And if to go for extrem optical power trial without much expectation, this would be your best choice:
650-2600mm Rokinon Actually upto 1300mm, IQ is rather good.
If you need the best IQ, bokeh etc, still having AF power, but a bit lower optical power, then look at
Minolta Beercan 70-210mm F4; Simply best for the price. I got mine for about $110.