There is no clear answer as to why Bob Ewell did not call a doctor. The cost implication is a plausible, if rather heartless reason. However, other testimony indicated that Bob was not known for giving consideration to his daughter. As a result there remains no evidence as to whether Mayella Ewell was raped by anyone. Her facial injuries suggest an assault by someone other than Tom Robinson - as Atticus attempted to prove - and the most likely offender was Bob Ewell. Again it is possible that the Ewells did not want the assault investigated becaus of this, but could also allude to Mayella's deep distrust of a community who treated her with pity at best.
We have to be careful of jumping to the same sorts of conclusions and prejudices as the people of the time, which is as implicit in the ambiguity surrounding Mayella's assault as it is in say, the two sides to Dolphus Raymond.
The episode you are talking about happens in Chapter 17 -- towards the very end of the chapter. You can look there for more details.
In this chapter, the trial of Tom Robinson is going on. In the part you're asking about, Atticus Finch is cross-examining Bob Ewell about what he did on the night he says that Robinson raped Mayella.
Ewell says he didn't go for a doctor because there was no need to do so. He also said he didn't want to have to pay the five dollars it would have cost for the doctor.
Even though Bob Ewell made the excuse to Atticus in the courtroom that he did not call a doctor because it was not necessary and that he had no money, his primary reason was because he needed time to think up a plan to get himself out of the mess he had caused. If he was drunk, which he probably was when he raped and beat his daughter, he needed a little time to sober up enough to be able to present the situation to the law exactly as he anted it to sound. He also had to have time to get it through his daughter's head what she would say and that she would say it! This thought is more inferred versus stated. I think it is the idea that Atticus was trying to expose.