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.
