Do you ai your own cows or does a semen company breeder come out and do it? If they come out and do it I would just have them keep them semen in one of their tanks, I'm sure they wouldnt charge that much for storage of the semen you wanted. Also do you know of anyone close to you or a friend that has a tank already, you could store it with their semen and I'm sure they wouldnt charge much for that, much cheaper than buying a tank.
But if you're dead set on buying a tank you can get one from MVE that'll hold all you need for about $500. And a smaller tank doesnt mean it'll be less expensive. Good luck!!!