Spaying and neutering larger rodents and rabbits is most cost-effective when they are young. How you measure cost-effectiveness is something we go into on this page, because cost is a factor of why people do not spay or neuter their pets.
Basic cost-effectiveness is taking the cost of the operation and dividing it up by the number of months you reasonably expect the animal to live. For example, if it is a young rat, start with 24 months. So a $120 procedure amounts to (the time value of money not included) $5 a month. Can you afford that? Probably.
If you keep only one sex, or else are completely responsible and keep males and females separate at all times, then do-nothing is a viable option for you. However, there are some health and behaviour benefits for spaying and neutering.
Females who are spayed at a young age are much less susceptible to mammary and/or uterine tumours. The cost of a tumour removal is commensurate with the cost of spaying, AND tumours often come back, so two operations can be assumed.
Sometimes neutering is required if you have a particularly aggressive or territorial male who won't get along with his cage mates, or who needs for his own good and your convenience to be integrated with other males. Or he could simply be a lucky fella because he'll have a female cage mate. (I know we like to anthropomorphize, but rats do not mate for life. Nonetheless, domestic rats do form pairs and the bond can be quite strong.)
If you think your rat is already pregnant…
Make a wise investment in yourself and your pet. If she could be pregnant, take her to the vet and see about spaying her. It is not too late until the vet tells you it is. If she is young, she will derive benefits by reducing tumour occurrences when she's older. If she's older, it could save her life from a difficult birth.
One way of telling early on if she's pregnant, well before she develops a distinctive pear-shape, is to weigh her daily on a postal scale. An increase in weight is a strong indication if you have reason to suspect she's been in contact with males.
The cost of spaying a rat, chinchilla or a rabbit varies; in Montreal, it's about $170. Remember: once a rat is spayed, you never have to worry about her getting pregnant again. All other alternatives mean it could be a repeat performance.
Rabbits have litters of 3 - 6 babies.
Chinchilla have only 1 - 2 babies, but remember - they live 15 years!
Rats have litters from 4 - 18 babies. Healthy moms usually have about 10 - 14.
The cost of a caesarian section should the birth go badly: $140 and up. Other assistance is additional, i.e. emergency hospital admission fees, because you don't get to choose the hour of birth. If a rat is in labour for more than 3 hours, she is in trouble! Do not wait!
The cost of euthanizing pinkies (termed culling), if you choose to go that route: $50 and up. Home culling is a crime. It is also very sad, as the mother will look for the babies she loses.
The cost of upgrading the cage size to accommodate Mama and 5 - 8 daughters: $100
The cost of a second cage or two to house 5 - 8 males at 5 weeks of age: $100
The cost of food and bedding for a period of six months, estimated duration to adopt all babies (except one daughter to keep Mama company): at least $75 more than before
Realistic number of rats at shelters that don't find homes because your babies found those homes first: 4 – and an excellent chance that one of your babies in turn has babies, and those babies end up in the shelter.
The average cost per rat in treating mycoplasma or removing tumours, likely and predictable diseases, over a lifetime: $200
Cost of euthanasia at a shelter - available whenever an animal needs it: $2 - 5, paid by the shelter as people do not usually offer or agree to pay or donate for this service
Some municipalities pay the shelter for this fee, and the shelter responds to this money by refusing to partner up with rescue organizations to save lives - because if they don't euthanize the animal, they don't get the fee. Shelters cannot run on rain, sunshine, and goodwill alone.
Cost of a much more humane (under sedative or anaesthetic) euthanasia at an exotic vet: $15 - 22 per small animal, paid by the client
So, if you don't spay your potentially pregnant rat, chinchilla, or rabbit, you are still spending money, and spending other people's money (or not spending money by subjecting the animal to cruelty). The chances of all babies making it into homes, where all their needs will be looked after all their lives, are not so good. Moreover, there are so many pets already living in this world, who deserve a better chance at being the pets they were supposed to be. You can help them first and foremost by not adding to their numbers.