I would do 20-25% down as non refundable deposit. Then probably give them 3 months max to pay it off. For someone like me on a college student budget i cant come up with that kinda money in a month. But paying it off over 3 months is no problem. Id also be willing to pay an extra $20 for the rats for those 3 months.
And as far as a steep price. I would say anything over $10K like aaron said deserves a 6 month plan. I believe most breeders do 20-25% down and 3 months max. Unless youre a repeat customer or you have something else worked out.