Your current rat will get terribly depressed if she does not have a companion. Is there a specific reason your rats get sick? Are they pet store rats? Why not try an esablished breeder with proven lines? If you'd like I can give you a few links. =)
As much as rat's love to have a partner, if you have one and can not afford bills for another, this means you just need to spend plenty of time playing and snuggleing with the one you have. After a bit rats will get used to being by themselves and introducing a new one can be hard (though not impossible). My advice , since you don't want to end up in a situation where all your babies are sick and you can't pay for treatment, is just to give plenty of love and care to your one baby now. Just think about rattie friends for the future and plan accordingly.
That's a sticky problem... Of course, you need to be able to afford vet care and the like. At the same time, rats are VERY social and NEED other rats. Humans can not replace rat friends.
Is this a situation where you had 3 and 2 passed on? It's common, and generally accepted that your two choices are to keep the one a lone rat or get more rats. Since you can't afford to care for more, then you really need to keep them a loner. You can't make up for not having rat friends, but lots of out time might help. The rat may have a shorter lifespan then it normally would, because rats can and do die of heartbreak, depression and loneliness.
If you can get another rat in the future, get young ones (two or more, preferably). Rats that have been kept alone CAN be introduced to other rats. Not all... but most. The best bet is to add young rats to the mix.
How much time do you spend with your rat a day? I have two girls that hate other rats being near them so I just make sure they get extra attention from me.
How long has your rat been on it's own with no cage mate? If it has been a while and he/she still plays and eats and is all around happy there is not much of a need to stretch your budget in case of emergency.
If you are there with her/him constantly it shouldn't be too much of a problem and as long as he/she seems happy and healthy I wouldn't push your budget. As soon as you do something will come up when it rains it pours with rats.
I am sure that this post will cause multiple retorts saying I am wrong and that is fine. I am simply giving my experiences.