Hello,
I don't think you'll find a drone with a 15-30 min flight time for $100. As the phantoms only have a 20-25 min flight time! Also any drone under $100 is going to be a mini or micro, much too small to chase birds away!
As some birds will actually attack or try to attack your drone if threatened, or protecting their nests! As it has happened to me with a pair of kitty Hawks, apparently I got too close to their nest, with my vision, and they started dive bombing me, forcing me to land, to save my bird, and also I did not want to injure them either, since I was invading them! My bad, but lesson learned!
On the other hand ( positive note )
Banggood.com, has a wltoys, V222 quad copter, for around $70 comes with camera ( I know you don't need it, but I think you would end up using it if you had it ) flies great, about 8-10 inches across, not big, but not small either, also has several attachments you can buy for it, missle launcher, water cannon, bubble blower, hook and basket, ect. 6-8 minute fly time, and can go pretty **** far before signal loss. Might work for your needs? You might need something larger, not sure which birds you are shewing away?
Personally, I would buy a crap load of ( bottle rockets ) and scatter bomb the fields, every hour or so! Eventually, they will stay away!
Lastly
Going to sound strange, but here goes!
We live near a lake, and across the lake, this one house had HUNDREDS of seagulls, on it and in the lake, everyday, crapping all over this guys house, so the old guy went and bought a record or tape, not sure, but it played, sounds of a hawk, killing seagulls ( or birds in general? ) he played it loudly everyday for about 1 1/2 weeks, now no birds! True story. Neighbors were mad at first, but happy now! I'd be glad to ask him what that tape was, and where he got it?
Hope this helps!
Good luck
J Dot