Some are more expensive than others. The two cases that take more time are vaccination series, and scheduled vaccinations, discussed below. Several types of inoculations require more than one shot, weeks apart. An example is your first cholera shot which requires a second shot about two weeks later. Typhoid and tetanus also require multiple injections several weeks apart, but only if you've never had one before.
Some vaccines are special and are only available at certain times.
When I wanted one of these, I was told that they gave that shot on the first Tuesday of each month. That's why you need to start early. It is not really a vaccine, but a preventative shot of Gamaglobulin. Some people get 'em, some do not. If you are headed directly into Malaria country, you should try to get at least a two to four week supply of malaria tablets from your doctor or the public health service.