The time of year will have a lot to do with which fish will be the cheapest.
Pollock is most likely a by product of crab fishing which would explain why it would be cheap this time of the year
I think you are safe on pollock but keep in mind a lot of saltwater fish are now being farm raised in pens which wouldn't make their diet too different than tilapia. I don't really see why farm raised fish would be bad for rays, if your really concerned about vitamin content you can always throw some vitachem in their diet