Unit Rate
The unit rate is the rate for one of something (Taken from HMHCO).
For instance, you could say that Bob can make 6 gift cards in 3 hours. The unit rate would be 2 gift cards per hour. We solved for this by dividing both values by 3.
Solving the Question
3 dollars per toy
This is a unit rate. Why? Because it gives us the rate, 3 dollars, and it is for one of something, which this case would be toys.
1 toy per 5 dollars
This is not a unit rate. Why? We still get the rate, 1 toy, but this time it isn't for one of something, which in this case is dollars. It is a rate for 5 dollars, not 1 dollar.
Answer
1 toy per 5 dollars