the particle 吧 has several uses, the simplest way to use it is to make suggestions. Ex: Then, let's do this way. (那,我们做这样吧). It is used also to indicate consent/approval, ex.: 好吧,我们一起去吃饭 (Ok, let's go eating together)
how to say "can or cannot afford to buy something"? It is enough to add the complement of result 得起 after 买. Ex. 我买得起这辆车 (I can afford to buy this car) or 我买不起这辆车 (I cannot afford to buy this car)