Hi paisley,
You can read more about the pros and cons of different types of mattress protectors in post #89 here.
If your limit is less than $50 then one of the options that use a thin semi breathable membrane would probably provide the best combination of ventilation and waterproof qualities for most people although they are not as breathable and can increase sleeping temperature more than the other two types (which are either water resistant or moisture absorbing rather than “waterproof”).
It depends on which of the properties of each different type are most important to you because none of them have the best of all worlds and there are always tradeoffs involved.
Phoenix